Updated WP.com Notification XML-RPC calls with JSON

Fixes: #919

Removed all of the remaining XML-RPC calls to WordPress.com for push notifications with REST-ish JSON calls.  Cleaned up the format of the code as well while I was in there for the methods I updated.  Still could use further work but that'll be a future thing.

New: Device ID is a numeric ID given by the back end after registering.  We're using this ID instead of token for all registration calls.  At some point we can probably forget that token since we no longer need it after registration is complete.