Refactor listener infrastructure (#35, #59, #63)

* Extract common listener behaviour from progress formatter
* Extract stats journal from progress formatter

Theses changes will ease the implementation of new listeners (e.g. the pretty, JSON and jUnit formatters).

lib/cucumber/listener.js

@@ -1,3 +1,38 @@
-var Listener               = {};
+var Listener = function () {
+  var self = {
+    hear: function hear(event, callback) {
+      if (self.hasHandlerForEvent(event)) {
+        var handler = self.getHandlerForEvent(event);
+        handler(event, callback);
+      } else {
+        callback();
+      }
+    },
+
+    hasHandlerForEvent: function hasHandlerForEvent(event) {
+      var handlerName = self.buildHandlerNameForEvent(event);
+      return self[handlerName] != undefined;
+    },
+
+    buildHandlerNameForEvent: function buildHandlerNameForEvent(event) {
+      var handlerName =
+        Listener.EVENT_HANDLER_NAME_PREFIX +
+        event.getName() +
+        Listener.EVENT_HANDLER_NAME_SUFFIX;
+      return handlerName;
+    },
+
+    getHandlerForEvent: function getHandlerForEvent(event) {
+      var eventHandlerName = self.buildHandlerNameForEvent(event);
+      return self[eventHandlerName];
+    }
+  };
+  return self;
+};
+
+Listener.EVENT_HANDLER_NAME_PREFIX = 'handle';
+Listener.EVENT_HANDLER_NAME_SUFFIX = 'Event';
+
 Listener.ProgressFormatter = require('./listener/progress_formatter');
+Listener.StatsJournal      = require('./listener/stats_journal');
 module.exports             = Listener;