Factor out invalid responses into a client error helper and return JSON formatted errors for consistency with success responses.
Factor out success responses into helper methods.
Add MsgOptions helper to make parse submitted payloads easier.
Fix up and split out reply_to tests.
Check invalid responses more thoroughly.
Add tests for bad content and to_addr in sent_to and reply_to and fix lack of checking in send_to.
Improve MsgOptions (notably adding more detailed error reports in the case where mutliple errors are found) and add tests for MsgOptions.
Merge branch 'feature/issue-770-better-message-sanity-checking' of github.com:praekelt/vumi-go into feature/issue-770-better-message-sanity-checking

Conflicts:
	go/apps/http_api/resource.py
Merge branch 'develop' into feature/issue-770-better-message-sanity-checking
Merge branch 'feature/issue-770-better-message-sanity-checking' into develop