Read consistency support for Datastore

[susanlinsfu]

It would be great if we could specifiy the read consitency like the java sdk can for google datastore.

[stephenplusplus]

Sounds good to me! Any interest in working on a PR?

[susanlinsfu]

I don't believe I am a good enough coder to implement this myself.

[stephenplusplus]

No problem, I've put together a PR in #1259 -- please share any thoughts!

$ npm install --save stephenplusplus/gcloud-node#spp--1218