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.



I agree, the direction we want to go is more plugin-style.  what we need to define is how plugins get found, get initialized, and get their config through the API.

There seem to be ways to have go's unmarshalling save "unknown" fields, but not as cleanly as, say, protobufs.  Or maybe there is a way to say "save all unknown fields in this map[string]interface{}" but I don't know how.