Add hasLayer to LayerGroup

Add the method hasLayer to LayerGroup, return false when not present or checking for a null layer.

src/layer/LayerGroup.js

@@ -40,6 +40,13 @@ L.LayerGroup = L.Class.extend({
 		return this;
 	},
 
+	hasLayer: function (layer) {
+		if (!layer) { return false; }
+
+		var id = L.stamp(layer);
+		return this._layers.hasOwnProperty(id);
+	}
+
 	clearLayers: function () {
 		this.eachLayer(this.removeLayer, this);
 		return this;