We want a more granular way of purging applications.  In particular, we'd like to be able to purge a *particular* application, as opposed to *all* deletable applications.

The changes I've made might violate a bit of the DRY principle - I guess this is because I wanted to change as little of the original django-evolution as possible.  I imagine it's going to undergo a big refactor soon anyhow.

All of the tests in the suite currently pass.  Couldn't find a test for purge, but I've tested it manually, and it seems to work alright.  purge-app does what I need it to do.