title: Trouble using Leafletjs MarkerClusterGroup and filters with Mapbox
body: <p>I've tried Mapbox and their API to create an interactive map. The purpose is to fetch points in a geojson file, and display them on the map. 
  They have to be filtered by their marker-icon and grouped depending on the zoom applied.</p>
  
  <p>I had no trouble using the MarkerClusterGroup plugin with leaflet and Mapbox, but I can't get the filters to work.</p>
  
  <p>This is my code :</p>
  
  <p><a href="https://gist.github.com/KuneStudio/5985864" rel="nofollow">https://gist.github.com/KuneStudio/5985864</a></p>
  
  <p>And this is the content of my json with the points :</p>
  
  <p><a href="https://gist.github.com/KuneStudio/5985858" rel="nofollow">https://gist.github.com/KuneStudio/5985858</a></p>
  
  <p>The markers are displaying correctly, the cluster part too, but I can't get the filters to work...</p>
  
  <p>Any idea?</p>
  
  <p>Thanks !</p>
  
  <p>(Note : using the console, I tried to display a log in the <code>map.markerLayer.setFilter(function(f) {}</code> , before the <code>return true</code>, but I have nothing showing up.</p>
  
  <p>Thanks again for your time</p>