[UTIL] Added a source code generator using the JSON API specs

The generator is based on [Thor](https://github.com/wycats/thor),
a library/framework for command line applications.

The generator will read the JSON API spec file(s), and generate
the Ruby source code (one file per API endpoint) with correct
module namespace, method names, and RDoc documentation.

It will generate a test file for each API endpoint as well.

Currently it only generates Ruby source, but can easily be
extended and adapted to generate source code for other
programming languages.

Usage example:

    $ thor api:code:generate ../../api-spec/*.json --force --verbose

rest-api-spec/utils/thor/generate_source.rb

@@ -0,0 +1,117 @@
+# encoding: UTF-8
+
+require 'thor'
+
+require 'pathname'
+require 'active_support/core_ext/hash/deep_merge'
+require 'active_support/inflector'
+require 'multi_json'
+require 'coderay'
+require 'pry'
+
+module Elasticsearch
+
+  module API
+
+    # A command line application based on [Thor](https://github.com/wycats/thor),
+    # which will read the JSON API spec file(s), and generate
+    # the Ruby source code (one file per API endpoint) with correct
+    # module namespace, method names, and RDoc documentation,
+    # as well as test files for each endpoint.
+    #
+    # Currently it only generates Ruby source, but can easily be
+    # extended and adapted to generate source code for other
+    # programming languages.
+    #
+    class SourceGenerator < Thor
+      namespace 'api:code'
+
+      include Thor::Actions
+
+      __root = Pathname( File.expand_path('../../..', __FILE__) )
+
+      desc "generate <PATH TO JSON SPEC FILES>", "Generate source code and tests from the REST API JSON specification"
+      method_option :language,  default: 'ruby',                                          desc: 'Programming language'
+      method_option :force,     type: :boolean, default: false,                           desc: 'Overwrite the output'
+      method_option :verbose,   type: :boolean, default: false,                           desc: 'Output more information'
+      method_option :input,     default: File.expand_path('../../../api-spec', __FILE__), desc: 'Path to directory with JSON API specs'
+      method_option :output,    default: File.expand_path('../../../tmp/out', __FILE__),        desc: 'Path to output directory'
+
+      def generate(*files)
+        self.class.source_root File.expand_path('../', __FILE__)
+
+        @input  = Pathname(options[:input])
+        @output = Pathname(options[:output])
+
+        # -- Test helper
+        copy_file "templates/ruby/test_helper.rb", @output.join('test').join('test_helper.rb') if options[:language] == 'ruby'
+
+        files.each do |file|
+          @path   = Pathname(file)
+          @json   = MultiJson.load( File.read(@path) )
+          @spec   = @json.values.first
+          say_status 'json', @path, :yellow
+          # say_status 'JSON', @spec.inspect, options[:verbose]
+
+          @full_namespace   = @json.keys.first.split('.')
+          @namespace_depth  = @full_namespace.size > 0 ? @full_namespace.size-1 : 0
+          @module_namespace = @full_namespace[0, @namespace_depth]
+          @method_name      = @full_namespace.last
+
+          # -- Ruby files
+
+          @path_to_file = @output.join('api').join( @module_namespace.join('/') ).join("#{@method_name}.rb")
+
+          empty_directory @output.join('api').join( @module_namespace.join('/') )
+
+          template "templates/#{options[:language]}/method.erb", @path_to_file
+
+          if options[:verbose]
+            colorized_output   = CodeRay.scan_file(@path_to_file, :ruby).terminal
+            lines              =  colorized_output.split("\n")
+            say_status options[:language].downcase,
+                       lines.first  + "\n" + lines[1, lines.size].map { |l| ' '*14 + l }.join("\n"),
+                       :yellow
+          end
+
+          # --- Test files
+
+          @test_directory = @output.join('test/api').join( @module_namespace.join('/') )
+          @test_file      = @test_directory.join("#{@method_name}_test.rb")
+
+          empty_directory @test_directory
+          template "templates/#{options[:language]}/test.erb", @test_file
+
+          if options[:verbose]
+            colorized_output   = colorized_output   = CodeRay.scan_file(@test_file, :ruby).terminal
+            lines              =  colorized_output.split("\n")
+            say_status options[:language].downcase,
+                       lines.first  + "\n" + lines[1, lines.size].map { |l| ' '*14 + l }.join("\n"),
+                       :yellow
+            say '▬'*terminal_width
+          end
+        end
+
+        # -- Tree output
+
+        if options[:verbose] && `which tree > /dev/null 2>&1`
+          lines = `tree #{@output}`.split("\n")
+          say_status 'tree',
+                     lines.first  + "\n" + lines[1, lines.size].map { |l| ' '*14 + l }.join("\n")
+        end
+      end
+
+      private
+
+      # Create the hierarchy of directories based on API namespaces
+      #
+      def __create_directories(key, value)
+        unless value['documentation']
+          empty_directory @output.join(key)
+          create_directory_hierarchy *value.to_a.first
+        end
+      end
+
+    end
+  end
+end