Adds Persistent Disk volume type.

Implements a GCEPersistentDisk volume type.

Persistent Disks can now be specified in pod manifests and will be mounted and exposed to containers.



Hm, looks like inlined maps just plain don't work at all: http://play.golang.org/p/NnwTh9KI5r

I'd say this is a bug in the json package. Should we fork it? Make a patch and try to get accepted upstream? I don't mind making the code changes to support this.