A quick and dirty fix for gapi not loading in time for ember-simple-auth

This is related to #9.

This commit is probably not something that should be merged into the main branch, but it makes it work for now, so I can continue work with implementing ember-simple-auth

I'll explain what's going on in the comments below, but here's the gist of it.

The initializer that loads gapi is called `requireGoogleLibraries`.

The load order for initializers in embger-gdrive is as following:

* `'requireGoogleLibraries'`
* `'googleDriveAuth'`
* `'store'`
* `'google-drive'` (set as `after: 'store'`)

`ember-simple-auth` has a single initializer which doesn't set a `before` or `after` option. In the global load order, with nothing changed, it get's loaded Before any of the `ember-gdrive` initializers. That's the **first problem**.

We can fix the load order in a quick and dirty way by doing this:
```
Application.initializer({
    name: "googleDriveAuth",
    before: ["store", "simple-auth"] // --- THIS IS THE CHANGE RIGHT HERE,
    initialize: function(container, application) {
      application.register('auth:google', GoogleDriveAuth, {instantiate: false});

      application.inject('controller', 'auth', 'auth:google');
      application.inject('route', 'auth', 'auth:google');
    }
 });
```
It still doesn't fix the issue. Here's the `'requireGoogleLibraries'` initializer:
```
Application.initializer({
    name: 'requireGoogleLibraries',
    before: 'googleDriveAuth',
    initialize: function(container, application) {
      application.deferReadiness();
      loader.load().then(function() {
        application.advanceReadiness();
      });
    }
  });
```
That's **the second problem**. It's asynchronous, so **it doesn't block loading of other initializers**. The quick and dirty solution will be commented on below.
This is the quick and dirty solution. 

I import loader and call `loader.load()`. It will return a promise, so I will authorize and then eventually resolve the promise `Auth.authorize` itself is returning only after everything has loaded. This makes `ember-simple-auth` wait for authentication.

Alternatively, I could go with loader.promise.then, since we have access to that directly as well, but it's really all the same. Ideally, this would be done in a nicer way. Maybe we should attach the loading promise the loader creates to the `gapi` object itself, somehow.

In any case, since the initializer that loads the `gapi` libraries is aysnchronous, I believe this is the only way to prevent an issue of authorization being attempted to soon. It's possible I'm missing something, though.

The alternative would be to make the `'requireGoogleLibraries'` initializer synchronous somehow, but I'm not sure that's an option, considering what it does.