Do not crash on unicode filenames

Introduce a new function, safe_walk, which wraps os.path.walk, but
ensures that the returned values are unicode objects.  This isn't
strictly safe, in that it is possible that some paths will not be
decodeable, but that case is rare, and the only alternative is to
somehow avoid all interaction between paths and unicode objects, which
seems especially tough in the presence of unicode_literals. See e.g.
https://mail.python.org/pipermail/python-dev/2008-December/083856.html

Testing Done:
Ran new test.

Reviewed at https://rbcommons.com/s/twitter/r/1193/
Sync of https://github.com/pantsbuild/pants/commit/8e5131d5ec8d8be91972997620c3b8b73c6b568b