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)

Maybe I'm misinterpreting how we'd use these criteria. Do you see us predefining a set of criteria and then a batch would just pick which ones to use? If so then yes, a many-to-many makes sense for that.