Add Labels and Annotations to MetadataAccessor

Justification of this PR:
I need MetadataAccessor to be able to read & write Labels from runtime.Object. Then I could get rid of type-switches, as in https://github.com/openshift/origin/blob/master/pkg/config/config.go#L104.
Note that we're storing pointers to the real map, and this function returns the dereferenced pointer. So I need to handle the `nil` ptr.

Do you want me to return `nil` instead of `map[string]string{}`, right?