Update push registration to use newer RESTful API on WP.com

Also adds a new parameter, a numeric device ID, to the registration process.
Update notification settings to use new REST API rather than XML-RPC
Updated unregister push to use REST API and Device ID
Updated save push settings to use REST API instead of XML-RPC
Fixed a problem with fetch settings - forgot to extract settings dictionary
Removed last traces of XML-RPC from WordPressComApi
Removed confusing boolean statement and replaced with a much clearer one :)
Merge pull request #1243 from wordpress-mobile/issue/919-note-settings-restful

Updated WP.com Notification XML-RPC calls with JSON