Test for setting a label on a story
Specify schema for a Pivotal story
Pivotal doesn't appear to return the labels field for a story that doesn't have any label info
so the PivotalAPI::Story ActiveResource model doesn't know about the labels field. Explicitly
specifying the schema with labels enables us to set labels on a story.
Indicate in comments that you can set labels
Only need to specify missing attribute - others are pulled from fetched instance by ActiveResource.