Fix documentation re: outgoing phase return value

* Returning False causes message processing to stop
Refactor logging in celery router

Send logging for celery router tasks to the 'rapidsms' logger rather than the 'celery' logger, and make sure celery knows the task failed by re-raising the exception.
Revert to using get_task_logger

We should still re-raise the exception, but it turns out `get_task_logger` does return a logger for the given module (not one inside celery).
Include id_ (a generated UUID) in payload to Vumi

Addresses issue #428
Update to match Vumi expectations
Add test that msg_id is sent to vumi
Merge pull request #429 from vkurup/feature/428-send-msg-id-to-vumi

Include id_ (a generated UUID) in payload to Vumi
Merge pull request #418 from vkurup/feature/417-outgoing-return-val

Fix documentation re: outgoing phase return value
Merge pull request #421 from tobiasmcnulty/develop

Refactor logging in celery router
prep for 0.17 release
0.18.0 is next release