Fix documentation re: outgoing phase return value

* Returning False causes message processing to stop
Link typo (rapidsms-generic)
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).
Merge branch 'release/0.16.0' into develop
Next release will be 0.17.0
Merge pull request #420 from jthurner/patch-1

Link typo (docs / rapidsms-generic)
Merge branch 'master' of github.com:rapidsms/rapidsms into develop
Merge branch 'develop' of github.com:rapidsms/rapidsms into develop
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