Push app context as part of application init

[rsyring]

So, apparently, when an app is instantiated it doesn't push context. This happens automatically when a request comes in and we do it manually for testing.

We sometimes have to create the app in alembic's env.py and, when we do this, we have to add the extra lines to push the context. Seems to me like Keg should just take care of that as part of init().

[rsyring]

Nope. See #90.