Use withId to fetch Site object for consistency

It's the pattern we use for posts and we should do the same thing
everywhere.
Use one main method for all API requests

Removes send_authorized_api_request; this way we don't have to worry
about handling private sites which need authenticated calls. All
requests now go through the sent_api_request method and the auth token
is added to the request if available.
Likes, Comments, Reblogs, and Related for Posts

Add get methods for fetching those objects for a post
Move client handling to WPCOM_REST_Object

No point in having each child object duplicate the effort
Static Factory Method for Me object

For consistency sake