Prepare for running multiple API versions on the same HTTP server
by decoupling some of the mechanics of apiserver. Define a new
APIGroup object which represents a version of the API.
The apiserver on initialization must be provided with a codec
for encoding and decoding all handled objects including api.Status
and api.ServerOp. In addition, the RESTStorage Extract() method
has been changed to New(), which returns a pointer object that the
codec must decode into (the internal object). Switched registry
methods to use pointers for Create/Update instead of values.
