make pep8 test routine reusable for other projects

Nothing changes for matplotlib test case but the routine can now be
imported and used from another project.
Also makes it better readable I think to have all the custom exclusions grouped
at the head of the file.
I also had to move the currently active manual skip to the bottom.
correct current skipping of pep8 tests for nose test discovery
Merge pull request #2486 from megies/reusable_pep8_testroutine

Make pep8 test routine reusable for other projects (work in progress).