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`.

Could also say `not properties or not update_properties` which eliminates tuple and list allocations (micro-optimization). Additionally, I would add
```py
assert not properties or not update_properties, \
    "{} {}".format(properties, update_properties)
```
That way there is a bit more context when this assertion fails.