dockerflow.logging.JsonLogFormatter should handle dates better #10

#8 fixes the exception that can happen when you log objects. What'd be even nicer, as @jezdez pointed out, would be to handle datetimes/dates : currently they are serialized as "datetime.datetime(2017, 4, 6, 9, 27, 7, 479427)" because that's the repr() for a datetime object, but we could do better by using DjangoJSONEncoder, which knows how to handle those objects and more.

Something like this should work, to test:

class SafeJSONEncoder(json.JSONEncoder):
    def default(self, o):
        return repr(o)

class SafeDjangoJSONEncoder(DjangoJSONEncoder, SafeJSONEncoder):
    pass

Then use cls=SafeDjangoJSONEncoder when doing the json.dumps.