Fixed a Django 1.5 compatibility issue with json/simplejson

In particular, this fixes the error "__init__() got an unexpected keyword
argument 'namedtuple_as_object'" that appears with Django 1.5.

simplejson and json are not entirely compatible in all versions. Python 2.6
and greater ships json anyway, so avoiding simplejson fixes the problem.
Add Luke Plant as a contributor.
Point 1.5 tox env to the RC1 tar file.

--HG--
branch : stable
Updating functional tests to better handle HTML assertions. See #85

--HG--
branch : stable
Clean up widget tests for HTML assertions. Fixes #85

--HG--
branch : stable
Merge in stable testing work.
Remove unused imports.
Removing deprecation warning for a feature long since removed.
Fix url imports to remove 1.5 deprecation warnings.
Remove another reference to simplejson in the tests.
Removing references to removed labels in the docs.