Check code style in tests

I'm thinking of adding code style testing using `flake8`.  This would ensure PEP8 compliance and add a ceiling to maximum code complexity per function.  We could ignore PEP8 rules that we don't agree with if we notice any that are annoying to obey.

The flake8 configuration would look something like this (in `tox.ini`):

    [flake8]
    ignore = E128
    max-complexity = 8
    max-line-length = 79

And the tox test environment configuration would probably look like this:

    [testenv:flake8]
    deps = flake8
    commands = flake8 simple_history

I'm not yet sure whether I think we should do this on Travis also.

Thoughts on testing code style using flake8?