Add support for instantiating user defined classes for couchdb documents.

[David Reid]

These should be lightweight objects, preferably with the property that there is only one instance per view of a document in memory at a time. (Similar to Storm (whose code we can't look at to do this)).

This may only be feasible for documents retrieved by opening a view. Unless we can come up with a good mechanism for hinting that at the class of an object.