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`.

Hmm. I think I meant to do

```python
    cls = super(JsonObjectMeta, mcs).__new__(mcs, name, bases, dct)
```

which just follows the super pattern rather than "hardcoding" the superclass. Any thoughts on which is better?