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.
Clean-up process_response.
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.
export conversation + test
import tool
Merge branch 'develop' into feature/issue-786-test-helpers-vumitools
Use new helper setup stuff.
Increased precision of decimal fields
Added traps to Decimal quantization operations
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