CouchDatabase onChange handler

[fabien]

I think it would be quite handy if the onChange block handler was public (currently resides in CouchInternal.h).

Either that, or populate the userInfo dictionary for kCouchDatabaseChangeNotification with the changed CouchDocument as well. However, I guess the notification queue won't allow this, unless the changed documents are queued up in an array as well.

CouchChangeTracker seems the way to go at the moment, but in that case, it's mandatory to expose its headers inside the framework as well.