title: How to set a Collection's url
body: <p>let's say I have :</p>
  
  <pre><code>var Book = Backbone.Model.extend();
  
  var Collection = Backbone.Collection.extend({
    model: Book,
    url: '/books',
    initialize: function(){
      this.fetch();
    })
  })
  </code></pre>
  
  <p>How can I change the <code>Collection</code>'s <code>url</code> when instantiating a new collection ?</p>
  
  <p><code>var AdventureBooks = new Books({ url: '/books/adventure' })</code> does not work</p>
  
  <pre><code>var AdventureBooks = new Books({ category: 'adventure' })
  </code></pre>
  
  <p>and in the Collection definition:</p>
  
  <p><code>url : '/books/' + this.category</code> does not work either.</p>
  
  <p>Thanks.</p>