Integrated ember-simple-auth into addon

This should resolve #16.

Everything is now moved into ember-gdrive.

~~One last thing I was hoping we'd avoid but ended up being necessary is accessing the containing application's config from within the addon.~~

~~The reason for this is that we need access to the GOOGLE_CLIENT_ID parameter. I'm currently looking into the best way for us to achieve this.~~

~~I believe doing it via an initializer is possible.~~

It was much simpler than expected.

This is something that `ember-cli-simple-auth` does in the exact same way:

https://github.com/simplabs/ember-cli-simple-auth/blob/master/app/initializers/simple-auth.js

Not the line where `Config` is being imported:
```
import ENV from '../config/environment';
```

Simply put, a relative path is used to import the app's configuration. This way the issue of knowing the app's namespace to import it's config is eliminated.

This was moved in entirely, from storypad. Other than in authentication, it is also used in storypad's document route to cache the login hint. Since that usage is also authentication related, I decided to simply change the import in that libary to point to ember-gdrive.