Merge pull request #194 from xyz666/fix_django_before_1_5_again

fix force_unicode and simplejson for django < 1.5
Option to add padding around the thumbnail.
tiny cleanup
fixed UnicodeDecodeError on tokey computing
Option to add padding around the thumbnail.
tiny cleanup
Option to add padding around the thumbnail.
tiny cleanup
Option to add padding around the thumbnail.
tiny cleanup
Merge branch 'dev' of github.com:xyz666/sorl-thumbnail into dev