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.

src/MarkerClusterGroup.js

@@ -1,4 +1,3 @@
-
 /*
  * L.MarkerClusterGroup extends L.FeatureGroup by clustering the markers contained within
  */
@@ -696,7 +695,17 @@ L.MarkerClusterGroup = L.FeatureGroup.extend({
 
 	_generateInitialClusters: function () {
 		var maxZoom = this._map.getMaxZoom(),
-			radius = this.options.maxClusterRadius;
+			radius = this.options.maxClusterRadius,
+            		radiusFunction = null;
+	
+	        //If we just set maxClusterRadius to a single number, we need to create
+	        //a simple function to return that number. Otherwise, we just have to
+	        //use the function we've passed in.
+	        if(typeof radius == "number") {
+	            radiusFunction = function(){ return radius };
+	        } else if(typeof radius == "function") {
+	            radiusFunction = radius;
+	        }
 
 		if (this.options.disableClusteringAtZoom) {
 			maxZoom = this.options.disableClusteringAtZoom - 1;
@@ -704,12 +713,12 @@ L.MarkerClusterGroup = L.FeatureGroup.extend({
 		this._maxZoom = maxZoom;
 		this._gridClusters = {};
 		this._gridUnclustered = {};
-
-		//Set up DistanceGrids for each zoom
-		for (var zoom = maxZoom; zoom >= 0; zoom--) {
-			this._gridClusters[zoom] = new L.DistanceGrid(radius);
-			this._gridUnclustered[zoom] = new L.DistanceGrid(radius);
-		}
+	
+	        //Set up DistanceGrids for each zoom
+	        for (var zoom = maxZoom; zoom >= 0; zoom--) {
+	            this._gridClusters[zoom] = new L.DistanceGrid(radiusFunction(zoom));
+	            this._gridUnclustered[zoom] = new L.DistanceGrid(radiusFunction(zoom));
+	        }
 
 		this._topClusterLevel = new L.MarkerCluster(this, -1);
 	},