Adds blinker signals.
Adds StoredObject::subscribe, plus unit tests.
Adds testing helper and a docstring #wwsd
Fixes requirements.
Fixes Travis link.
Allows ForeignField references by class or by name.

[Resolves #76]
Reorders tests.
Merge branch 'develop' into feature/callbacks
Merge branch 'feature/callbacks' into develop
Fixes regression on null backrefs.
Adds record-level validation to schema definition.