Move *Builder classes outside AVExtensions::Builder module

Avoid including these classes on FormBuilder.

lib/simple_form/action_view_extensions/builder.rb

@@ -1,9 +1,8 @@
 module SimpleForm
   module ActionViewExtensions
-    # A collection of methods required by simple_form but added to rails default form.
-    # This means that you can use such methods outside simple_form context.
-    module Builder
-      class Builder
+    # Base builder to handle each instance of a collection of radio buttons / check boxes.
+    # Based on (at this time upcoming) Rails 4 collection builders.
+    class BuilderBase #:nodoc:
       attr_reader :object, :text, :value
 
       def initialize(template_object, object_name, method_name, object,
@@ -23,20 +22,25 @@ module SimpleForm
       end
     end
 
-      class RadioButtonBuilder < Builder
+    # Handles generating an instance of radio + label for collection_radio_buttons.
+    class RadioButtonBuilder < BuilderBase #:nodoc:
       def radio_button(extra_html_options={})
         html_options = extra_html_options.merge(@input_html_options)
         @template_object.radio_button(@object_name, @method_name, @value, html_options)
       end
     end
 
-      class CheckBoxBuilder < Builder
+    # Handles generating an instance of check box + label for collection_check_boxes.
+    class CheckBoxBuilder < BuilderBase #:nodoc:
       def check_box(extra_html_options={})
         html_options = extra_html_options.merge(@input_html_options)
         @template_object.check_box(@object_name, @method_name, html_options, @value, nil)
       end
     end
 
+    # A collection of methods required by simple_form but added to rails default form.
+    # This means that you can use such methods outside simple_form context.
+    module Builder
       # Create a collection of radio inputs for the attribute. Basically this
       # helper will create a radio input associated with a label for each
       # text/value option in the collection, using value_method and text_method