Attempt to fix ENV is undefined issue in Auth module

This should fix #7 

Disclaimer: It doesn't work.

Here's what I tried

1. Left everything in Auth.reopenClass, changed clientID into a property ( triedboth volatile and non-volatile).
2. Left everything in Auth.reopenClass, changed clientID into a property, retrieved clientID via this.get().
3. Same as nr. 1, but moved everything into Object.extend and returned instance of Auth.
4. Same as nr. 2, but moved everything into Object.extend and returned instance of Auth

For both cases, I also tried adding a regular function instead of a property, and then called the function inside the authenticate method, forcing ENV.GOOGLE_CLIENT_ID to evaluate only when authenticate is called.

Also using a fixed constant, not touching ENV at all, but if there was a property present, it would fail.

In all cases, as soon as I added a property (or a function), a breakpoint inside the property/function or inside the authenticate method never gets hit anymore. Some sort of strange interaction I couldn't get a tag on would cause the whole thing not to get evaluated at all. At that point, the session variable inside storypad is null, and authentication doesn't work at all anymore.

If I discard all of this the effect is that storypad reports an error due to ENV being undefined at start, but after that, there's a second pass through it, everything is loaded correctly and the application works properly.

Since converting ember-gdrive to an ember-cli addon would give it more or less direct access to config/environment, we would not have to rely on window.ENV at all and I believe this issue would be completely avoided. Because of that, **I suggest we leave it the way it is and focus on converting it to an ember-cli addon, since in the current state, nothing is really broken**. There's just a console error that doesn't affect execution.

Here's a link to an ember-simple-auth issue that touches on the subject:

https://github.com/simplabs/ember-cli-simple-auth/issues/21

Tried this, tried without volatile, tried with a function:
```
getClientId: function () {
  return ENV ? ENV.GOOGLE_CLIENT_ID : '';
},
```

I even tried just returning a simple fixed string. 

It's always the same issue - breakpoints inside this code or inside the authorize function never get hit. 

If I leave it as a simple property (`clientID: ENV.GOOGLE_CLIENT_ID,`) then it works both as `Object.extend` and as `Auth.reopenClass` just fine, except the old error is being thrown once at start.