Add ability to use a function for maxClusterRadius

I find it's necessary to use a different radius at different zoom levels. This patch lets you continue to use a single number for all zoom levels, but also lets you use a function to vary the radius for each zoom level.

So you can do this:

```
     L.markerClusterGroup({
            maxClusterRadius: function(zoom) {
                var zoomToRadius = { 
                    7: 50,
                    8: 50,
                    9: 50,
                    10: 50,
                    11: 40,
                    12: 30,
                    13: 20,
                    14: 10,
                    15: 1,
                    16: 1
                }
                return zoomToRadius[zoom];
            },
            //...
     });
```

Or keep doing this:

```
     L.markerClusterGroup({
            maxClusterRadius: 50.
            //...
     });
```