Add support for Python3 (fixes #10)

I used futurify to add Python 3 support. Aside from the auto-generated import changes, only a few things needed to be changed:

• Add __hash__ operator to Node class
• Encode strings as utf-8 before sending to hashlib.md5
• Change test_repr to use proper string representation.

In addition, the following features have been added:

• Test multiple python versions using Tox
• Make setup.py test DTRT
• Add setup.py classifiers
• Cache ffmpeg download on travis

Fixes #10 (closed).