Move dict-to-json conversion in response from app to response class' force_type().
Make pylint happy.
Convert carafe into an app/extension repo.

- Remove carafe.factory and carafe.core
- Remove carafe.ext.json but keep jsonify in carafe.utils.
- Minor formatting changes.
Use class attribute for signaler.
Update README with changes.
Better clean targets.
Release v0.3.0.