Remove server property from endpoints

These two should be isolated, since we have better methods to handle
passing data between them now.
Remove ResponseHandler interface

This doesn't make sense to keep around any more, since it's not passed
to endpoints. Instead, we can require anyone to subclass WP_JSON_Server.
Don't pass the server instance to endpoints

Final step. :)
Merge pull request #543 from WP-API/two-remove-response-handler

Avoid passing server to the controller each time