title: Backbone Association not working with fetch
body: <p>I am using Backbone-Associations from 
  github-Backbone associations
  <a href="https://github.com/dhruvaray/backbone-associations" rel="nofollow">https://github.com/dhruvaray/backbone-associations</a>
  for nested models to work with backnone.</p>
  
  <p>I am getting an error while calling fetch() on a model. the response json from server exactly matches the model definitions. i cant figure out what i am missing...</p>
  
  <p>Versions:</p>
  
  <p>Backbone: 0.9.2</p>
  
  <p>Backbone-associations: 0.2.0</p>
  
  <p><strong>TypeError: this.attributes[relationKey].off is not a function
  this.attributes[relationKey].off("all");</strong></p>
  
  <pre><code>//this is the code i am using..
  
  var ProductVariation = Backbone.AssociatedModel.extend({
      defaults: {
          ImageUrl:'',
          Desc:''
      }
  });
  
  var Product = Backbone.AssociatedModel.extend({
      relations: [{
          type: Backbone.Many,
          key: 'ProductVariations',
          relatedModel: ProductVariation
      }],
      url: '/GetProductById',
      defaults: {
          CategoryId: 0,
          ProductVariations: [],
          Summary: ''
      }
  });
  </code></pre>
  
  <p>am i missing something? any help appreciated... </p>