Add Labels and Annotations to MetadataAccessor

Alright. So, do you want me to change this struct to
```
type TypeMeta struct {
  api.TypeMeta
  api.ObjectMeta
}
```
instead?

Justification of this PR:
I need MetadataAccessor to be able to read & write runtime.Object Labels. Then I could get rid of type-switches, as in https://github.com/openshift/origin/blob/master/pkg/config/config.go#L104.