Fix-up outbound resource and add tests for new methods.
Add docstrings.
Better parameter checking.
Change a ContactsResource docstring (null instead of None)
Merge branch 'develop' of github.com:praekelt/vumi-go into develop
Factor out sending failures a bit.
Use _mkfail for errback too.
Add support for null content in replies.
Merge branch 'develop' into feature/issue-420-fix-up-outbound-resource
Merge branch 'feature/issue-420-fix-up-outbound-resource' into develop
ZipFile is only a context manager in Python 2.7. (sdehaan)
Merge branch 'develop' into feature/issue-237-travis-builds