Eliminate all pep8 and most pylint warnings

Repo now down to zero pep8 violations and 23 pylint warnings (http://jenkins.edx.org:8080/view/All/job/xblock-feature-branch-tests/139/)

100% of the warnings are docstrings that I am not sure how to write or TODOs. I promise to follow up with responsible parties to take care of these things.

@nedbat can you review?

@rocha would you mind reviewing at minimum the files in xblock/{content, problem, slider, structure}.py? These appear to be toy XBlocks that you created. I added in a lot of docstrings but would appreciate that you review these docstrings and make sure they're accurate.