Strip STATIC_URL, leading / from proxied URLs

STATIC_URL often contains host or generic_host (esp. if STATIC_URL is a
path on the same domain), so it needs to be removed first to ensure it
is removed completely. Also removed leading '/' from URL, since it
appears staticfiles doesn't like absolute paths.
New sites in README
Merge.
Added a function mezzanine.utils.urls.next_url which is used to retrieve redirect URLs from a request's next param, while verifying that the redirect URL is valid.
Fix min Dajngo version
Use request.get_host rather than request.META['HTTP_HOST']
Fix Django version for travis