Support datastore on local development server

[ryanseys]

https://cloud.google.com/datastore/docs/tools/devserver

[stephenplusplus]

Should we accept configuration to a Dataset's instantiation?

var ds = gcloud.datastore.dataset({
  host: "localhost",
  port: "8080"
})

[stephenplusplus]

Just leaving a note for the future implementer of this, I needed to set the Content-Length header to avoid a 411.

authorizedReqOpts.headers['Content-Length'] = pbRequest.length;