add a test case that checks for pep8 compliance

 - among other things, this will make it easy to spot pep8 pollution in
   pull requests before merging
 - currently won't work because it depends on changes in matplotlib
   (see matplotlib/matplotlib#2486), have to wait and see how that turns
   out

See http://tests.obspy.org/9690/#1 for a test example of the current master.

I also want to find out untracked files and exclude them if git is available via a system call. But I'll wait if the idea gets accepted in matplotlib, because I'll do it there then.