Default to Python 3

In support of #8.

Right now I've just installed it and I'm trying to understand the right way to default to python3. There is the option of symlinking both `python` and `pip`.
I enjoyed [this thread with Fernando on their mailing list](https://groups.google.com/d/msg/pystatsmodels/ReZ54tv6KSw/2dBAzvM5csEJ) about Python 3.2 support.