Add support for Python 3

This PR adds support for Python 3.

Fixes: https://github.com/wearpants/twiggy/issues/26

This PR builds upon the tox and Travis support that I added in https://github.com/wearpants/twiggy/pull/33, because tox and Travis are super useful for ensuring that Python 3 support works and that I didn't break support for Python 2.

Passing Travis CI build: https://travis-ci.org/msabramo/twiggy/builds/21557026

Tox output:

```
$ tox
  py26: commands succeeded
  py27: commands succeeded
  pypy: commands succeeded
  py33: commands succeeded
  congratulations :)
```