Plugins will have their own ability classes for their resources.
However, their hooks which execute in the main_app context will need to
access them. This commit provides away to do that. (#231 is almost there)
If the model and the engine are in the same module, this makes auth!
from the main application work reliably when used in the controllers for
engine resources. Should have no effect on main application's auth.
Continuing work on #231 ...