Add lint target

Pulling parts of #104 out to this separate PR based on comment by @lawrenceakka in  https://github.com/petteraas/SoCo/commit/f1e917a5cf0d2bf90ef94798bd92f30bb7a5adc3

If the problem is mostly related to travis+pylint, I think we should still run flake8 as is, and either -E on pylint or no pylint at all?