Refactor Config labels, fix #286

1.    Fix #286: Fix Config with nil Labels
    
    - Fix nil Label case
    - Remove mergemap.MergeMap() dst auto allocation
    - Improve mergemap tests and its readability
    - Re-enable AddConfigLabels test for Unknown Generic Object having Labels field

1.    Refactor Config obj.Labels to use meta.Accessor

1.    Move mergeMaps() into its own pkg as `pkg/util/mergemap`.MergeInto()

1.    Add Labels and Annotations to MetadataAccessor
    
    cherry-pick(github.com/GoogleCloudPlatform/kubernetes): 28f5d51a5f6975a575d0cd5157c114878deba2ed

1.    meta_test should not depend on runtime.TypeMeta
    
    cherry-pick(github.com/GoogleCloudPlatform/kubernetes): 508724b1f8431b860f3c35607127a41e70f2f0ed
You don't need to repeat `1 << iota` in this and next line, Go compiler is smart enough to figure it out. All you need to tell it how to construct first constant and it'll handle the rest of them.