Coordinate with Vumi team to complete RapidSMS Relay

``RapidSMSRelay`` is in the [feature/issue-302-rapidsms-relay](https://github.com/praekelt/vumi/tree/feature/issue-302-rapidsms-relay) branch. Pull requests [303](https://github.com/praekelt/vumi/pull/303) and [447](https://github.com/praekelt/vumi/pull/447) are tracking these changes.

We need to:

* Finish bulk messaging enhancements.
* We've started docs to setup a test SMSC. We need to finish these and add anything else that would be useful.