Bug 1030972 - Add ability to auto-import tasks from Bugzilla

Here are some tentative changes to the models, for feedback. Please see inline comments for some highlights. Diagram attached.
![importschema](https://cloud.githubusercontent.com/assets/6024326/3616075/f98f30ca-0dce-11e4-9437-51864ec0da18.png)

Yes, I think this is a bug with South and generating migrations, but I doubt it's going to get fixed as South is being deprecated in favour of Django migrations (which I think are coming in 1.7).