Syndicate thumbnails for XMLRPC Push

* Remove old thumbnail cruft
* Replace with new hotness to properly syndicate thumbnails attached to posts (requires Syndication plugin be active on both sites as it uses a custom endpoint)
* Remove internal success and error tracking for clients (this was causing problems with the ability to do subsequent XMLRPC calls: edit/new post then send thumbnail)
* Client actions should now return an ID, bool, or WP_Error

fixes #33