Add subdocument support

[bojand]

http://developer.couchbase.com/documentation/server/current/sdk/subdocument-operations.html
http://developer.couchbase.com/documentation/server/current/developer-guide/sub-doc-api.html

Not sure exactly how this would look / work and what kind of API, shortcuts to provide.
We could also try and be smart about changes somehow and automatically use it in document saving.