Use builtin json instead of django.utils.simplejson deprecated in 1.5

For further explanation: https://docs.djangoproject.com/en/1.5/releases/1.5/#system-version-of-simplejson-no-longer-used
django.conf.urls.defaults is deprecated
Merge pull request #110 from williambr/master

 Use builtin json instead of django.utils.simplejson deprecated in 1.5