storage: createReadStream: accept start/end offsets

RE: #318 

This adds `options.start` and `options.end` to `file.createReadStream`, using the `Range` HTTP header when calling the API.

// @janesconference for a review, since it was all his idea :bulb: :+1:
Is this `res` (and the one at [R351](https://github.com/GoogleCloudPlatform/gcloud-node/pull/327/files#diff-8ce1c5ba96fca8cd68637b6416287596R351)) being used anywhere?