flake8 fixes
Sort imports

`isort taggit/**.py tests/**.py`
setup.cfg: add flake8 section
tox: pass '{posargs}' to runtests.py
Merge pull request #253 from blueyed/tox-posargs

tox: pass '{posargs}' to runtests.py
Merge pull request #252 from blueyed/flake8

Flake8 fixes / sorted imports
Raise ValueError with invalid types in `add`
get_prefetch_queryset, get pk val, rather than named attr

fixes problems with prefetch_related when tags are on a base class/model.
includes unit tests of problem/fix.
migrations for django 1.7, remove Model from names