JIRA backend

Provides an abstraction over the issue tracking importer such that issues can come from anywhere, and implements a JIRA backend based on [jirasurvivor](https://github.com/gengo/jirasurvivor/) (courtesy of @shawnps).

I've simplified the data model, removing a couple of properties that didn't seem necessary (e.g. user email).

Note that I haven't included the jirasurvivor theme, but I intend to do that next.

Comments on the JIRA implementation, and the overall approach, are appreciated!

Ping @alecsloman, @dannymidnight