Linting and PEP8 compliance
- [ ] update docs with tools to use & that Travis is checking

I have been away from the codebase for a while and ouch, my eyes. I have used pyflakes or flake8 assiduously for Python development for a long time now and it seems that conventional style isn't being followed in the codebase:

$ flake8 pandas/core | wc -l
2047

It's a big codebase so adding a pre-commit hook might be too onerous, but can we get flake8 back to zero (with some well-placed `# noqa` marks on unintentionally-unused-imports)?

Secondly, perhaps we should add a Readability or Style label for these kinds of issues?
