Add a staticfiles finder for Pipeline assets

When using runserver with DEBUG=False, e.g. when running tests, you
can't find Django-Pipeline generated assets. This finder module will
allow those assets to be served.

Usage:

    STATICFILES_FINDERS = (
        "django.contrib.staticfiles.finders.FileSystemFinder",
        "django.contrib.staticfiles.finders.AppDirectoriesFinder",
        "pipeline.finders.PipelineFinder"
    )