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
Version bump to 1.4.15
Added tag 1.4.15 for changeset a24985814970
Update auto-generated changelog, requirements.
Revert broken static proxy change.
Better fix for static proxy urls.
Version bump to 1.4.16
Added tag 1.4.16 for changeset 29f28b33f710
Update auto-generated changelog, requirements.
Synch transifex translations for cset a9946f4b68dc
fix a KeyError that would occur if _update_cache is set on a response that has no content type
Fix possible unicode errors on blog import. Closes #789.
Merge
Added John Henry to AUTHORS