Allow additional exclusion patterns for PEP8 task

I found that the pep8-exclude option overrides exclusions in the pep8.rc file. 
* The hardcoded south_migrations exclusion in `run_pep8.py` overrides settings that are added to pep8.rc
* Added `PEP8_EXCLUDES` option which allows additional exclusions to be added to the pep8 task.
* Exclusions can be either list or tuple format (it coerces to a tuple).