Topic directive is responsible for adding callback=JSON_CALLBACK to URL

The topic controller does not know that jsonp is used for loading topics, so it should not be responsible for appending "callback=JSON_CALLBACK" to the URL. Instead, the topic directive should be responsible for that. This PR fixes that. It also adds a gaUrlUtils service, providing an "append" function for adding parameters to a URL. Other utility functions can be added later.

This is ready for review.