Added json serialisation for datetimes

Hey guys,

I've added a DatetimeSerializer class to utils.py, which can be used to serialise datetimes (and dates/times etc) to JSON in the request. 

I've also added a sample test case (and made the test script executable). 

Have you thought about running your test case automatically using something like [Travis CI](about.travis-ci.org/docs/user/languages/python/)?