Watch delivers current state for resourceVersion=0

Allows clients to get the current state without having to execute
a get followed by a watch.  Makes integration with action loops
much cleaner.

Split from #846
Well, couldn't we also state that someone can define arbitrary serialization into etcd, so watch should support arbitrary deserialization?  Whereas versioning is coupled to the codec AND to the object that is created.  I guess we could version twice but that feels yucky.  Should transform be able to override version?  After all, versioning in etcd wraps the object we store, so I can set an arbitrary object and know it will have a version.