title: Magento - get filterable attributes by category
body: <p>I have created a custom navigation module specifically for a website, but I really want to be able to list filterable attributes by a specific category. So for instance my main navigation is:</p>
  
  <ul>
      <li>Category 1</li>
      <li>Category 2</li>
      <li>Category 3 etc.</li>
  </ul>
  
  <p>I then that when a user mouses over a category, they are then presented with an expanded menu with a few filterable options e.g.:</p>
  
  <hr />
  
  <h2>Category 1</h2>
  
  <p><strong>View by manufacturer:</strong></p>
  
  <ul>
      <li>Manufacturer 1</li>
      <li>Manufacturer 2</li>
      <li>Manufacturer 3 etc.</li>
  </ul>
  
  <hr />
  
  <p>I am able to get all filterable attributes for the store, but I want this list to pull in only the filterable attributes per category, as for instance Category 1 may have different manufacturers to Category 2. I then need to cache these results as this will not change often.</p>