Add tests for opt_out store.
Convert message id to unicode before storing in opt out store to handle messages that have yet to be round-tripped via JSON (this should probably go straight to vumi.messages).
Fix opt_outs_for_addresses so that it now handles empty addresses lists.
More explicit protection against non-unicode message ids.
Merge branch 'feature/issue-717-fix-address-retrieval-for-empty-groups' into develop