title: backbone.js cache collections and refresh
body: <p>I have a collection that can potentially contain thousands of models.  I have a view that displays a table with 50 rows for each page.</p>
  
  <p>Now I want to be able to cache my data so that when a user loads page 1 of the table and then clicks page 2, the data for page 1 (rows #01-50) will be cached so that when the user clicks page 1 again, backbone won't have to fetch it again.</p>
  
  <p>Also, I want my collection to be able to refresh updated data from the server without performing a RESET, since RESET will delete all the models in a collection, including references of existing model that may exist in my app.  Is it possible to fetch data from the server and only update or add new models if necessary by comparing the existing data and the new arriving data?</p>