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
Merge pull request #284 from mariocesar/improve-filters

Improve filters: fix SuspiciousOperation in regex filters