put mock WSGI app state in objects for each test module
use context manager for test install/uninstall

allows us not to use reset(), just get a fresh InstalledApp
reduce InstalledApp boilerplate

test modules don't have to subclass BaseInstalledApp
merge MockWSGIApp into InstalledApp for simpler use
Merge pull request #12 from sashahart/test_overhaul

Test overhaul to reduce global state.