Support JSONP callbacks for the API/v1 endpoints

SeaGL currently uses OSEM as a backend, but still has a static website
serving as our public interface. The API endpoints have a great amount
of data, but without the jsonp callback paramater, embedding the data
is a bit difficult. This change just adds the jsonp callback param
to all the API endpoints. This should be a NOOP for anyone using direct
json calls, but will allow cross domain requests that require jsonp.

app/controllers/api/v1/tracks_controller.rb

@@ -7,7 +7,7 @@ module Api
       def index
         tracks = @conference ? @conference.program.tracks : Track.all
 
-        respond_with tracks
+        respond_with tracks, callback: params[:callback]
       end
     end
   end