added support for simplejson and ujson

Added support for simplejson and ujson.

Sometimes you must use json (ex: talking to the browser or want a human readable format).

Then if you want to squeeze every bit of performance there's the drop-in replacement simplejson and also ujson (slight less compatible).

ps: importlib requires 2.7+ (which is a safe assumption these days) but there's also a backport in pypi