Support Python 3

**This is broken, not ready for merge.**

This PR is for discussion on ongoing work on moving to Python 3. I'll comment again when I hit a stable milestone. Please ring in if you see anything happening that looks bad.

@JoshData, the most tricky stuff here is the unicode processing during download in utils. I'll take a look at your work in `congress-legislators`, though feel free to take the reins on that if you feel motivated.