Use DjangoJSONEncoder instead of a custom Encoder

It's better to use built-in django JSON encoder, it's there since django 1.3 so doesn't need backwards compatibility code.

Fixes #324
Use json library instead of simplejson

Its better to use the built-in json library.

see https://github.com/django/django/blob/1.5/django/utils/simplejson.py
Fixes import priority
Merge pull request #356 from camilonova/patch-1

Use DjangoJSONEncoder instead of a custom Encoder