Support latest RapidSMS version

I revived the RapidSMS Vumi backend created while in Rwanda last year and merged it into our ``feature/bulk-messaging-api`` branch. You can view the current [Vumi backend docs](https://rapidsms.readthedocs.org/en/feature-bulk-messaging-api/topics/backends/vumi.html) to see how to set it up in RapidSMS.

Here are my current changes to Vumi's ``feature/issue-302-rapidsms-relay`` branch based on what's new in RapidSMS. This works for me, but I'm not certain it's the best implementation! There are a few things I'd like to support, but I'm not sure I know how to implement them in the RapidSMS relay side:

* Sending 1 message to many recipients. Right now the Vumi RapidSMS backend sends a list of recipients in JSON, but the RapidSMS Relay only sends the message to the first recipient. What's the best way to send a bulk message from within the RapidSMS relay?
* It's possible for RapidSMS to send back any context originally posted to RapidSMS for the incoming message, such as the message ID, etc. Would it help to send any of this back so Vumi knows if it's a reply?

Looking forward to adding official support for Vumi in RapidSMS!