catch exceptions during sending with BlockingRouter; update exception handling on Celery/Database router to match
update docs to mention how exceptions are handled
Merge branch 'develop' into feature/blocking-router-catch-exceptions

Conflicts:
	rapidsms/router/celery/tasks.py
	rapidsms/router/db/tasks.py
raise MessageSendingError in send_to_backend
don't relog exception
list proper functions in __all__
Merge pull request #355 from rapidsms/feature/blocking-router-catch-exceptions

Catch exceptions in blocking router during send