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`.

Yeah so that's one of those things that is carried over from couchdbkit: if you wrap JSON that has properties not listed in the schema, it automatically converts strings in particular formats to the their respective types. This PR takes a step towards loosening that hardcoded behavior, so that if you want, for example, you can set

```python
    class Meta(object):
        string_conversions = ()
```

and then _no_ dynamic string conversions happen.