Make ATpy into an affiliated package

In my view, ATpy should be deprecated in the long term once all the functionality is in Astropy, but in the mean time, we can make it a package that uses Astropy as the single main dependency (except h5py and SQL stuff). This PR makes ATpy into an affiliated package that is provided for legacy value.

I've managed to get all the tests running on Travis, including the SQL. The docs will follow in the next pull request. These will include a migration guide to Astropy.