Check for URL path clashes in wagtailimages caused by a pre-wagtail-0.3 urlconf, and fail loudly if one is encountered
Merge pull request #519 from gasman/feature/fail-on-wagtailimages-urlconf-clash

Check for URL path clashes caused by pre-Wagtail-0.3 urlconfs that try to import wagtail.wagtailimages.urls as /admin/images/