fix #309 when THUMBNAIL_PRESERVE_FORMAT=True and remote images
Merge pull request #310 from mgaitan/fix309

fix #309, fix _get_format for images with urls
Update compat.py
Update current.py
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
Merge branch 'dj1.7' of https://github.com/mariocesar/sorl-thumbnail into dj1.7
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