REST API Client V1.1

Add the ability to call the REST APIs V1.1 on WordPress.com.

* New wpcom REST Endpoints, available on version 1.1, are needed to implement Calypso Stats on mobile.
Just noticed the line below, which is setting the user agent. Shouldn't this use [WordPress.getUserAgent](https://github.com/wordpress-mobile/WordPress-Android/blob/feature/REST-Client-v1-1/WordPress/src/main/java/org/wordpress/android/WordPress.java#L660)?