This is the first set of tests for the extension framework classes.  There'll be more, don't worry.

I'm using Mock here (http://pypi.python.org/pypi/mock/0.6.0), so you'll need to install it with easy_install to run
these tests.

I've also removed the manager argument in the ExtensionInfo constructor, since the manager is assigned to an Extension automatically by ExtensionManager (extensions/base.py line 333).

Open to all suggestions / criticisms / feedback.

Note:  not sure why it's not included in this diff, but I've also added a test/ folder to extensions, with admin_urls.py and __init__.py.  This lets ExtensionTest pass.

All Djblets tests pass.