Resolves issue #1396 supporting srcset

allows you to use image_tag helper and have it build the asset urls for
your srcset images, just like it does for your main src. Leaves absolute
urls alone, (having // in the path)

```
<%= image_tage 'pic_1980.jpg', srcset: 'pic_640.jpg 2x, pic_1024.jpg 3x' %>
=> <img src="/images/pic_1980.jpg" srcset="/images/pic_640.jpg 2x, /images/pic_1024.jpg 3x">
```

middleman-core/features/image_srcset_paths.feature

@@ -0,0 +1,7 @@
+Feature: Support srcset property as params for image_tag helper
+  This lets you specify responsive image sizes
+
+  Scenario: Rendering an image with the feature enabled
+    Given the Server is running at "image-srcset-paths-app"
+    When I go to "/image-srcset-paths.html"
+    Then I should see '//example.com/remote-image.jpg 2x, /images/blank_3x.jpg 3x'

middleman-core/features/markdown_kramdown_in_haml.feature

@@ -33,9 +33,10 @@ Feature: Markdown support in Haml (Kramdown)
       :markdown
         [A link](/link_target.html)
 
-        ![image](blank.gif)
+        ![image](blank.gif){: srcset="image_2x.jpg 2x"}
       """
     Given the Server is running at "markdown-in-haml-app"
     When I go to "/link_and_image/"
     Then I should see "/link_target/"
+    Then I should see "/images/image_2x.jpg 2x"
     Then I should see 'src="/images/blank.gif"'

middleman-core/fixtures/image-srcset-paths-app/image-srcset-paths.html.erb

@@ -0,0 +1 @@
+<%= image_tag 'blank.jpg', srcset: '//example.com/remote-image.jpg 2x, blank_3x.jpg 3x, http://example.com/remoteimage.jpg 4x' %>

middleman-core/lib/middleman-more/core_extensions/default_helpers.rb

@@ -255,5 +255,18 @@ class Middleman::CoreExtensions::DefaultHelpers < ::Middleman::Extension
       url = url_for(url, options)
       super
     end
+    
+    # Modified Padrino image_tag so that it finds the paths for srcset
+    # using asset_path for the images listed in the srcset param
+    def image_tag(path, params={})
+      params.symbolize_keys!
+      
+      if params.key?(:srcset)
+        images = params[:srcset].split(',').map {|size| (size.include?('//') ? size : asset_url("images/#{size.strip}")) }
+        params[:srcset] = images.join(', ')
+      end
+
+      super(path, params)
+    end
   end
 end