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 
As I was commenting before, if the media has already be downloaded, `self.post.featuredImage` should be populated, and we should be able to use that here. Or we should remove the `featuredImage` attribute from Post if it's not being used anymore.