add imexam as affiliated package

This does just what it says: adds `imexam` to the registry of affiliated packages.

A few questions for @sosey before this is ready to be merged: 

1. I've classified this as "not stable", which just means that it's under active development.  Do you agree with that, or do you think it should be considered "stable".
2. Is there a PyPI entry?  Would you be willing to put it up there so people can ``pip install`` it?  It's fine if it's just an initial "development version".
3. You've had a chance to look over the description at the bottom of http://affiliated.astropy.org, right? So you know what I mean here by "provisional"?

cc @astrofrog