Issue: #73 - GenericHTTP actor.

* Unify mock_tornado() and tornado_value() helper functions during testing.
Add docs and dry test to GenericHTTP
Exceptions instead of False
Fix unit tests.
Add failed response test.
Add integration tests
Merge pull request #113 from Nextdoor/issue_73

Issue: #73 - GenericHTTP actor.
Add BaseActor execution conditions.