Give geometrues a centroid method.

src/api/geom/collection.js

@@ -128,6 +128,6 @@ ol.geom.Collection.prototype.remove = function(components){
         this.removeComponent(c);
         return true;
     }, this);
-    
+
     return this;
 };

src/api/geom/geometry.js

@@ -24,3 +24,12 @@ ol.geom.Geometry.prototype.bounds = function(opt_arg) {
         return this.getBounds();
     }
 };
+
+/**
+ * Returns the centroid of the geometry.
+ *
+ * @returns {ol.geom.Point} The centroid of the geometry.
+ */
+ol.geom.Geometry.prototype.centroid = function() {
+    return this.getCentroid();
+};