title: accessing $resource instance url in angularjs
body: <p>I created resource instance with this code:</p>
  
  <pre><code>$scope.book = Book.get({bookId:1});
  </code></pre>
  
  <p>Here is service code:</p>
  
  <pre><code>var bookServices=angular.module('bookServices',['ngResource']);
  
  bookServices.factory('Book',['$resource',
      function($resource){
          return $resource('/books/:bookId',{},{});
      }]);
  </code></pre>
  
  <p>Book has cover image, accessible by url:
  /books/:bookId/cover.png</p>
  
  <p>How can I get url from resource instance?</p>
  
  <pre><code>$scope.book.$$url 
  </code></pre>
  
  <p>is undefined.</p>