Add Python3 Support

I mostly just added a bunch of encoding, especially around the HTTP
Digest Auth stuff.  This commit also adds six as a dependency.
Add a setup.py

This allows tox to run, and allows me to install httpbin as a depenency
(see #136)
Add Tox

Tox is a utility that helps you test on different versions of Python.
Merge pull request #137 from kevin1024/master

Python3 Support