fixes #38 return key/prop combined object.

This currently only patches `.get` & `.getAll` to return a single array of combined key/property objects. I will move on to `query` if this seems headed in the right direction :+1: 
results.length could never be < 0. https://github.com/GoogleCloudPlatform/gcloud-node/blob/master/lib/datastore/index.js#L130

Could you please fix it while you're there.