Adds support for custom serializers

This commit implements the 0.9.x style of explicitly passing a serializer to a JSON render call in a controller as an option to `render :json`

So now we've got backwards compat for this:

    render json: @post, serializer: CustomPostSerializer

I also added support for passing the adapter explicitly - which could come in handy.

   render json: @post, adapter: RadAdapter

Additionally, this commit adds Yard for docs.