Allow to set the layer style

By firing a simple CHANGE event, we can redraw the layer when
a style is assigned.

src/ol/layer/vectorlayer.js

@@ -261,6 +261,17 @@ ol.layer.Vector.prototype.getStyle = function() {
 };
 
 
+/**
+ * Set a style for this layer.
+ * @param {ol.style.Style} style Style.
+ */
+ol.layer.Vector.prototype.setStyle = function(style) {
+  this.style_ = style;
+  this.dispatchEvent(
+      new ol.layer.VectorEvent(ol.layer.VectorEventType.CHANGE, [], []));
+};
+
+
 /**
  * Returns an array of features that match a filter. This will not fetch data,
  * it only considers features that are loaded already.