Set the backed to 'dropbox' and start the authentication flow in order
to obtain an API token from Dropbox.
function () {
    // TODO handling when token is already present
    this.rs.setBackend('dropbox');
    if (this.token){
      hookIt(this.rs);
    } else {
      this.rs.authorize({ authURL: AUTH_URL, scope: '', clientId: this.clientId });
    }
  }