Updated documentation re AirTrans and models.py

Hey Aaron,

Just did a test run on an AirTrans flight, and (as I thought) the script doesn't work.  I'm already checked in so I can't debug (oops) but I at least added a note to the README file about it, and added a Known Issues section (one of my favorite tools when I can't fix something - makes it a Known Issue, not a bug ;-).  I also added some definitions to the entities in models.py a few months ago when I was reading through it that are included in this pull request.

If you pull this, our branches will be completely synced. :)

Grant