Revisit identifiers spec

Partially addresses #1135.

This is the result of discussions in the past few days about imprecision and confusion. Hopefully simpler. There are a few FIXME comments in the doc - discuss.
If we're going to mention namespaces here, then I think we should reconcile with #1114.

Kubelet should not alter the namespaces of pods that go through the apiserver, since they should arrive with a namespace as per #1114. The apiserver should set a default namespace in the case that one is not provided.

So, in the case of a pod created from a file on the Kubelet, I think it's ok for Kubelet to assign a namespace using a hash or whatever if one is not specified. OTOH, I also think it should be legitimate for a file to specify a namespace, such as one derived from the minion's hostname and owner. Ensuring that such namespaces don't collide would be the responsibility of the cluster admin.