Python 3 compatibility
No doubt there are ways this could be improved, but this gets it installing, importing, and _almost_ passing the test suite.

There's one test failure left, and I think that's something odd with numpy, specifically the `ndarray.searchsorted()` method. It returns seemingly random results when it's called in `panel.get_axis_dummies()` - but I can't replicate those results calling searchsorted on identical arrays interactively.

It doesn't add any failures to the Python 2 test suite (one test is failing for me in master), but I've only checked this with Python 2.7.
