3.0 - Clean up dead/broken code

Clean up some of the dead/broken code in CakePHP. I've tried to remove code based on a few things:

1. Model & ClassRegistry - These have been replaced by the new ORM.
2. Things that depend on 1. Features like DbAcl and behaviors have been removed to reduce confusion around what things will work in any future release.
3. Scaffold - This class is tightly coupled to the old Model and needs a re-think/re-write anyways. Perhaps the crud plugin would be a good candidate replacement.
4. Update other tests. I've skipped a bunch of tests because they also rely on the old Model code. I've tried to ensure there are issues for the skipped tests.

I apologize for the size of this diff but this is the smallest I could make it and have the remaining tests pass.