Retain empty docs/_static folder

Without docs/_static, `tox -e docs` fails with:

Warning, treated as error:
WARNING: html_static_path entry u'(path)/django-simple-history/docs/_static' does not exist
PEP8 fixes and ignores

tox -e flake8 now returns w/o errors
Merge pull request #136 from jwhitlock/tox_fixes

Tox fixes