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`.

I like how this is coming out in layers. Yes, you're totally right that there needs to be some mechanism for deciding whether a dynamic string should be treated as a string or as some other type. This is done by setting `string_conversions` in `Meta`; if you want to see what that looks like, check the default used in the `JsonObject` class definition (within this PR). Keep asking questions!