Add flake8 to build script

Although I ran PEP8's reindenter once on the code months ago, we're slowly diverging. Fix this once and for all by integrating a style checker into the travis tests.

I cheated on the files that @bmyerz actively develops; I didn't want to much with what he's working on.

I also removed some unused code. We can always get it back with `git revert` if we need it.