Switch (most) go.vumitools tests to helpers.
Merge branch 'develop' into feature/issue-786-test-helpers-vumitools

Conflicts:
	go/vumitools/tests/test_metrics_worker.py
Merge branch 'develop' into feature/issue-786-test-helpers-vumitools
Remove stray prints.
Merge branch 'develop' into feature/issue-786-test-helpers-vumitools
Update some new tests from develop.
Merge branch 'develop' into feature/issue-786-test-helpers-vumitools
Comment updates.
Merge branch 'develop' into feature/issue-786-test-helpers-vumitools
Use new helper setup stuff.
Better TODOs.
pep8 fixes.
Cleaner (and better documented) return mechanism for stored message helper methods.
Get maybe_async_return() from vumi instead of making it ourselves.
Merge branch 'feature/issue-786-test-helpers-vumitools' into develop