Split resources.py into per-resource files.
resources.py has become unmanageable, so it's now split up into a bunch

of different files.
The problem going into this was the interdependency between resources.

At class definition time, they neatly form a tree, but at request

handler time, there are circular dependencies.
The solution to this is to have a Resources class that dynamically loads

the right resources from the right places upon request, and caches

them. All references to resource instances go through this, and it

handles dishing them out. It also handles registering them against

models.
The logic for resources haven't changed. It's just code movement. The

only major change here is the logic in resources/init.py.

Unit tests run.
Docs build.
Web UI works.