Add support for custom User model

Adds support for use of a custom User model in django 1.5 while still
maintaining backwards compatibility with 1.4 and 1.3.
Merge pull request #291 from fusionbox/feature/django-1.5-compat

Add support for custom User model
adds tox test matrix
removes dependency for django-setuptest and adds more flexible invocations for tox
makes travis use Pillow instead of PIL
fix my email
separate testing for django-1.5c1 and django-dev
use unique temp folder for all temporary files

(so multiple instances of the test suite can be run in parallel)
mention PIL for running test suite. mention detox.
adds screenshots to README
readme screenshots 2
screenshots
Add Retina support to filer when using easy thumbnails
more prominently show that PIL needs JPEG support
PEP8 cleanup
Changed function signature get_thumbnail_name to be compatible with easy_thumbnails supporting hires images
Update filer/admin/permissionadmin.py

Added some admin options to the folder permissions to improve usability.
Update filer/models/imagemodels.py

Removed checking for staff user when serving images 
with permissions through filer. This should be like it is 
with regular files and folders.
Merge pull request #300 from zombat/develop

Improve usability in Permission Admin and fixed download permission issue
Merge pull request #299 from jrief/retina-display

Add Retina support to filer when using easy thumbnails