use stdlib json module whenever possible to make it work with django 1.5+
fix templates to be compatible with django 1.5+
allow gargoyle to depend on django 1.5+
fixed failing tests

some tests dependend on features not available in Python 2.6. Other test
failed due to api changes in django-modeldict 1.4. Fixed both issues to
get tests pssing in all supported dependency combinations.
Merge pull request #70 from ricardokirkner/master

make gargoyle work with Django 1.5