title: How to add customer "total number of orders" and "total spent" to order grid in magento 1.7
body: <p>I am wondering how to add the following two extra colums in the order grid in magento 1.7</p>
  
  <ul>
  <li>Customers total number of orders</li>
  <li>Customers total amount spent on orders</li>
  </ul>
  
  <p>I have managed to add the colums, but I can't get it to show any data. I believe the key to the problem is in the function *_prepareCollection()*.</p>