Allow to refresh a source and reload its data.

Provide an API to refresh a source and reload its data in an associated
layer, as for example already loaded tiles in a ol.source.Tile source
are not refreshed. Additionally a test for the new
ol.source.Source.refresh() function is provided.

Currently internal state is not cleaned up, e.g. already loaded tiles
in a `ol.source.Tile` source are still cached and not reloaded at all.

github issue #4867 (https://github.com/openlayers/ol3/issues/4867)

src/ol/source/source.js

@@ -150,6 +150,15 @@ ol.source.Source.prototype.getWrapX = function() {
 };
 
 
+/**
+ * Refreshes the source and finally dispatches a 'change' event.
+ * @api
+ */
+ol.source.Source.prototype.refresh = function() {
+  this.changed();
+};
+
+
 /**
  * Set the attributions of the source.
  * @param {Array.<ol.Attribution>} attributions Attributions.