Auth plugins

This supercedes #1358. 

Since some folks want a very simple default implementation for auth, and others want to be able to plug in a more full-featured implementation, I've focused on the problem of defining how Authorization plugs in.

In the process, I found I was unable to talk about that without also defining how Authentication and Group Membership should plug in.  

I've also removed the text from access.md about "userAccounts", since I no longer feel that is resource type has to be part of kubernetes.
If authentication was disabled in the apiserver in lieu of an authenticating proxy, would group membership still be able to live in the apiserver?