Baker assistant API:
- Changed the `IBakerAssistant` interface to pass baker/page objects
  to both being/end methods to make it easier for users to implement
  assistants. `onPageBakeStart` also now takes a `BakeResult` object
  instead of just a `bool`.
- Added unit tests.
- Changes to mock objects to support new unit tests.
Added data provider API.
Added unit tests.
Ability to specify several fixup callbacks for the PieCrust configuration.
Updated CHANGELOG.