Factory the DocumentSnapshot web service structure

The DocumentSnapshot lacks factories: from DocumentModel and to DocumentModel. Currently, the DocumentModel is read and a DocumentSnapshot is created through protected methods in NuxeoRemotingBean. My proposal would be to move the implementation in the DocumetnSnapshot itself, as a true method factory. A need of detecting existence of blobs could be also thought (as for synchronization).
The factory updating a DocumentModel from the DocumentSnapshot (at least for non blobs properties, but it would be easy to add blobs support too) can be also added.