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`.

Ah, thank you, `string_conversions` was a critical connection I had not made yet. Would it make sense to add `string_conversions` to the `Meta` class below to provide a more complete example that demonstrates all the pieces needed for a successful persist/load round-trip?

What if someone happens to put a string that successfully passes through a string conversion into a field that is supposed to remain a string?