test notebooks for pep8

[gregcaporaso]

came across this this morning - would be really cool if this could just run in the background on all cells (e.g., during testing, it wouldn't be good to run on normal execution as it'd confuse users who were editing code, but not strong python programmers).

[jairideout]

This may also be useful.

[gregcaporaso]

We'll now need to do this either on the markdown files, or on the generated notebooks after the build process.