Make APLpy into an Astropy affiliated package

This PR incorporates the Astropy affiliated package template into the code and also brings APLpy up-to-speed with Python 3. I've removed code that's no longer needed (e.g. the logger and the py.test runner) and I've added a .travis.yml file so that the tests will now run on Travis (note there are no actual image tests yet, only checking that the code runs).

(don't merge yet, as I need to double check a couple of things)