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.
Yes


> On Nov 29, 2014, at 4:40 PM, Vojtech Vitek <notifications@github.com> wrote:
> 
> In pkg/api/meta/meta.go:
> 
> > @@ -288,6 +324,28 @@ func (a genericAccessor) SetSelfLink(selfLink string) {
> >  	*a.selfLink = selfLink
> >  }
> >  
> > +func (a genericAccessor) Labels() map[string]string {
> > +	if a.labels == nil {
> 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?
> 
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