DocumentSchema inside a Document instance.

This may have something to do with the clone issue.

```
>>> from couchdbkit import *
>>> class Address(DocumentSchema):
... address = StringProperty()
...
>>> class Person(Document):
... name = StringProperty()
... address = SchemaProperty(Address())
...
>>> p = Person()
>>> p.name = 'XXX'
>>> p.address.address = 'XXX ADDRESS'
>>> p.name
u'XXX'
>>> p.address.address
u'XXX ADDRESS'
>>> p2 = Person()
>>> p2.address.address
u'XXX ADDRESS'
>>> p2.name
```

As you can see, because I am declaring Address() in the constructor of SchemaProperty, any new instances always point to the same object.
I have tried to get around this by trying to make a new Address object in **init**, but I cannot seem to get it to work.