Make Registry._components a reified property

Now it's not necessary for subclasses to do a super() call just to initialize
_components.
Adjust doctest config so all doctests run

There are some issues with using relative package paths in doctests.
E.g., __package__ isn't always set where it normally would be.

Remove util.load_doctests() since it's no longer used and we don't want
to maintain it anyway.

We still can't get rid of our test runner because py.test still doesn't work
with packages like tangled.mako and tangled.sqlalchemy.
Add Registry.contains(key, differentiator)

This is nicer than `[key, diff] in registry` in some cases
Add Registry tests