Add GCS bucket labels feature

[swcloud]

No description provided.

[stephenplusplus]

We currently support setting any property of the Bucket resource in storage.createBucket():

storage.createBucket({
  labels: {
    myLabel: 'my label value'
  }
}, function(err, bucket) {})

If a user wants to update the labels, they can use bucket.setMetadata():

bucket.setMetadata({
  labels: {
    myLabel: 'my new label value'
  }
}, function(err) {})

Do we need more than this?

[stephenplusplus]

Got an update on what is wanted:

• bucket.deleteLabels([labelName, ...], function(err) {})
• bucket.getLabels(function(err, labels) {})
• bucket.setLabels({ labelName: 'labelValue' }, function(err) {})

@danoscarmike does this look correct?

[danoscarmike]

Yes, looks right to me. @jskeet for vis.