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 
This method should be moved to MediaServiceRemoteREST, since it's used to parse JSON. Also it's a good opportunity to review `Media` methods and delete or move methods like `updateFromDictionary:` or `XMLRPCDictionaryForUpdate`. 

Or I can take care of those in #2601.