Add WP.site static convenience method for creating new sites

The [WP.com API client](https://github.com/Automattic/wpcom.js) provides a useful `wpcom.site()` method, for constructing a client instance bound to a particular url. This seems like a convenient shortcut and will be familiar for people coming in from that API, as well as a nice abstraction for any prospective users who aren't familiar with the `new` syntax, so I've added .site() as a static method on the WP object.