Adding className option for each marker cluster.

Knowing full well the iconCreateFunction option exists for customizing cluster markers, I added a simplified className option that allows for a simple class name to be added to markers.

For our use case, we only want to change the colors of the markers based on category so we set multiple cluster groups with different classes, similar to this:
```javascript
var clustersBg = L.markerClusterGroup({className:'marker-cluster-background'});
var clustersPr = L.markerClusterGroup({className:'marker-cluster-preparation'});
var clustersUn = L.markerClusterGroup({className:'marker-cluster-underway'});
var clustersDb = L.markerClusterGroup({className:'marker-cluster-debriefing'});
```
This is cleaner than writing our own function for creating the DivIcon.