Brute force remove files from temp that don't want to be removed.

Fixes the git error in Issue #64.

Failure was due to files in `temp` being marked as read only.  This http://stackoverflow.com/questions/1213706/what-user-do-python-scripts-run-as-in-windows was a nice start, but the individual files needed to be changed - doesn't appear there's a flag to do that, so manually walk the tree.  http://stackoverflow.com/questions/7228296/permission-change-of-files-in-python

The cleaner way to do this is to define a function for `rmtree` to call on failure, but I only ever get this on the git directory and not the hg, so unless it pops up elsewhere this ugly little kludge should fix things.
Could you `pragma: no cover` this? (means this block won't count towards coverage).

Thanks for this, sure is annoying there isn't a force kwarg!