Implement Google Datastore API

[sashaafm]

Add a wrapper for the Google Datastore API.

[peburrows]

Whether or not you want to pull in another dependency is your choice, but I'll mention here (as I did in #3) that diplomat is under active use and development, and I released v0.1.0 last night which switches to the v1beta3 API and adds support for a few other things (full changelog here, for reference).