Exim Python integration

This is a tracking PR for code-review purposes, so that I can look over Todd's work with GitHub's tools, since I've gotten sucked into that workflow.  All this code is by @mrballcb and I haven't looked at it yet.

Pushed branch with:
```sh
git push github +refs/remotes/tlyons/master_volatile_python:refs/heads/tylons_python
```