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.
We can't have it both ways - I can not have this spec leave namespacing out of teh main portion and simultaneously say that kubelet can't apply namespaces.

If we start here, and #1114 flies, then we hoist namespace up and rejigger the rules a bit.  Or I can go back to saying that all objects have a namespace string, and let #1114 be the decider of that string.