Fix normalize_paths to allow whitespace

This fixes normalize_paths so that it strips whitespace before and after
paths before working on them. This allows you to do specify excludes
with multiple lines in the config file.

This also tweaks normalize_paths so that the behavior matches the
docstring.

This also adds tests.

Fixes #339
I called this UtilTestCase figuring this is a good place to test some of the other functions in `pep8.py` that have no tests. If you want me to name it something else, I'm game.