Drop use of simplejson

Since json is a standard library as of Python 2.6 and 2.6 is our
baseline target we can drop the need for simplejson.

I ran the tests outlined in http://stackoverflow.com/questions/712791/what-are-the-differences-between-json-and-simplejson-python-modules

The results are very similar for Python 2.7 but Python 2.6 is much slower.

Regardless of that I think this should land because:
1. It works fast for what will most likely be our largest userbase Python 2.7 and up
2. We get to use a standard library.
3. We know we don't have any Unicode issues that simplejson apparently won't fix, though I haven't checked what those are specifically.