normalize paths in match_location

This solves issue [219](https://github.com/cyberdelia/django-pipeline/issues/219) for me.

The problem in `match_location(self, name, path, prefix)` is that while `name` comes from the settings and is written with forward slashes, `path` comes from the static files finder and could be written with backward slashes (on Windows for example).

We need to normalize them before we can compare them textually.