Python 3 already!

This (finally) implements Python 3 support, using the six module. Currently, all tests pass on Py3 and Py2. However, I'm waiting on two things before merging this:

- iminuit (required for `fit_lc`) doesn't yet support python 3. Currently the `fit_lc` test is simply skipped if iminuit isn't installed. I'd like to get iminuit working on Py3 and ensure that fit_lc works.

- sncosmo still has only about 50% test coverage, so I'm worried that some untested things still don't work. Gotta up that test coverage first...