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.

Just a simple import-export for now. if we need to extend it, we should be able to export a custom mixing which extends this mixin.