GAE prod and dev support for datastore

Fixes #104.
Looking back on this, we can use defineProperty to allow dynamic analysis of the env vars:

```js
var gae = {};

Object.defineProperty(gae, 'info', {
  get: function() {
    if (!process.env.GAE_LONG_APP_ID) {
      return null;
    }
    return {
      id: process.env.GAE_LONG_APP_ID,
      isProd: process.env.GAE_VM !== undefined
    };
  }
});
```