WIP: tighten identifier spec 

DO NOT MERGE YET

There are still a couple open questions.  As I read through it I wanted to wordsmith a bit, which lead to reordering some to read sort of top-down.  There were a couple things I thought were wrong (pod name should be a subdomain) and a few things I think we can make more concise.  I also added examples and clarifies corollaries.

@smarterclayton in particular


I'm not sure we want to support this - We don't currently (as far as I can tell) actually return 201 Created and pass back a locator, so the client can never know what they just did.  This means they can never clean up what they just did.

Do we want to actually DTRT and return 201 + a locator?  If so, I can get behind this more flexible model.  Otherwise it feels like an opportunity for users to make a mess