model only rake environment

With the introduction of cache_classes, rake tasks can now be caching the models, controllers, and helpers.

It would be nice to have different levels of the environment rake dependency that loaded different levels of these such that things like db:migrate would only have to load up the models, not the entire tree.

This would speed up many of the rake tasks' startup time. In a large application this makes a decent different in rapid development.

Having controllers load before migrations happen can also cause chicken and egg problems with controllers seeking out table info on initialization purposes for caching purposes when they weren't expecting to be loaded.