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 this VIP ticket:

https://wordpressvip.zendesk.com/agent/tickets/32623

and also some slack reported bugs for setting the featured image.

cc @koke