Added missing method getEventProperties in Core

lib/timeline/Core.js

@@ -396,6 +396,16 @@ Core.prototype.getCustomTime = function(id) {
   return customTimes[0].getCustomTime();
 };
 
+/**
+ * Retrieve meta information from an event.
+ * Should be overridden by classes extending Core
+ * @param {Event} event
+ * @return {Object} An object with related information.
+ */
+Core.prototype.getEventProperties = function (event) {
+  return { event: event };
+};
+
 /**
  * Add custom vertical bar
  * @param {Date | String | Number} [time]  A Date, unix timestamp, or