Use native json (for django >= 1.5) instead of django.utils.simplejson

Steve,

This is a fix to make django-piston compatible with Django 1.5's JSONEncoder and subclasses. I've pretty much modelled this on https://bitbucket.org/jespern/django-piston/pull-request/25/compatibility-fix-for-json-emitter-with/diff.

This makes tests pass in PS when the json imported into PS is similarly amended. I have pinned PS develop branch (Django 1.3.7) to the previous commit, to avoid that specific backwards incompatibility.

Nick