* Fix loading of periodic tasks and clean up extension loading.
This does a few things:
- add tests for extension loading
- refactor the extension and periodic task loading
- better handle assertions raised by extensions (e.g. when an extension tries to override an already registered view)
- attach exception traceback to error log during loading for improved debugging
* Use site.addsitedir instead of calling pip.
* Use sys.path instead of site.addsitedir and also the setup.py egg_info command.
