Add RestartPolicy to Pod and PodTemplate

This is the first PR to add RestartPolicy to Pod. Next PRs will hook policy with kubelets. 
I must not be making myself clear... I'm saying it's very inconvenient to plumb an []error all the way through the system, so it should be wrapped in something that converts it to an ordinary error. Here's a very rough example: http://play.golang.org/p/r7IOmElEhU