Set up tox for mimic

This runs unit tests on python 2.7 and pypy, and runs doc linters, doc tests, and does better linting of the code.

Also updated the documentation to reflect using tox to develop, and updated the travis config file to use tox.
Is there a bug that we could track in Travis, so we know when this should be removed? I think that if this lands as-is there ought to be an issue for it so we don't propagate copy-pasting this shell script into every python project in the world…