Type configs

This is a modest restructuring that makes all of a JsonObject subclass's type inferences configurable. It's done like this

```python
class MyJsonObject(JsonObject):
    class Meta(object):
        # this replaces the default DateTimeProperty with your custom one
        # you can also completely override all properties from scratch with
        # properties = {...}
        update_properties = {datetime.datetime: MyBetterDateTimeProperty}
```

Now, in a `MyJsonObject` subclass, you'll still be able to have `DateTimeProperty`s, but in the case of a dynamic property (one that you didn't specify in the schema) it will now default to using `MyBetterDateTimeProperty`.

We store all UTC datetimes as timezone-naive throughout all our code, and that's the convention used here. I think it'd be pretty hard and mostly fruitless to change that everywhere, no?