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
Allow overriding records manager class in register

By calling register(Model, records_class=Foo), a user can choose a
different class than HistoricialRecords to use for the manager.  A silly
example is included for tests.  A more serious example is at:

https://github.com/jwhitlock/web-platform-compat/blob/master/webplatformcompat/models.py#L38
Merge pull request #136 from jwhitlock/tox_fixes

Tox fixes
Change style of setting records_class default
Merge pull request #135 from jwhitlock/override_manager_in_register

Allow overriding records manager class in register