Feature/media services implementation

Implement MediaService using REST and XMLRPC for getMedia and uploadMedia.

This allows featured images to be set using the REST API and also to retrieve them one by one instead of depending of the sync of the full media library.

It will solve some reported bugs for setting the featured image.

cc @koke 
We generally use `dispatch_async` instead of NSOperationQueue. In any case, the completion block should be called from the queue associated to the managed context using `performBlock:`