fix SuspiciousOperation in regex filters

The SuspiciousOperation is raised here if the read of the image gets
permissioned denied. In many cases where this is wrong it is because the
path given is /media/... which may refer to MEDIA_ROOT not /media.
make regex filters replace MEDIA_URL prefix in paths

MEDIA_URL prefixes are replaced with MEDIA_ROOT
Merge branch 'master' of github.com:mariocesar/sorl-thumbnail into filters
Merge branch 'master' of github.com:mariocesar/sorl-thumbnail into filters
fix hash in tests filter tests
add test config for django 1.7
Update compat.py
Update current.py
add django.setup() in test runner for django1.7
Update compat.py

optimizing condition of image format
Update current.py

Optimizing condition of image format
Merge pull request #314 from renecarvalho/suport-png-mixin

Suport png mixin in django admin
Update test settings and utils for Django1.7

 - Set package name explicitly on INSTALLED_APPS. Fixes "Item" import
   issue.
 - Add MIDDLEWARE_CLASSES to suppress warning.
 - Add "enter" keyword to util sending ``setting_changed`` signal:
   https://docs.djangoproject.com/en/dev/releases/1.7/#signals
 - Change ``mimetype`` to ``content_type``
   https://docs.djangoproject.com/en/dev/releases/1.7/#features-removed-in-1-7
Merge pull request #318 from yourcelf/dj1.7

Update test settings and utils for Django1.7
Fix installed apps in tests for Django < 1.6
Update readme with info about django 1.7 support
Merge pull request #308 from mariocesar/dj1.7

Support for django 1.7
Merge pull request #284 from mariocesar/improve-filters

Improve filters: fix SuspiciousOperation in regex filters