[#1] Fix Django 1.7 compatibility

Replace import of django.utils.simpljson with import of json from std
lib, but keep the simplejson name in the code
[#1] Add check for Django version when importing simplejson

Use django.utils.simplejson if Django version < 1.5
[#1] Fix json import in sites.py

Do the json import in a similar way as the previous commit, checking
for the version of Django.
Merge pull request #2 from akvo/feature/1_django_1.7_compat

[#1] Fix Django 1.7 compatibility