Use 'tox' to run tests

[jml]

Allows for testing multiple Pythons.

• make sure tests pass
• update changelog
• use six

Builds on #107

[leth]

Regarding six: I suggest you vendor it into the project, rather than adding it as dependency.
The whole world tends to depend on six, which creates a version constraint satisfication nightmare.

[jml]

Thanks! We don't yet need six (according to our tests), so I'm skipping that for now.