Implement 'pong' command to hybi.js

lib/websocket/driver/base.js

@@ -96,6 +96,10 @@ var instance = {
     return false;
   },
 
+  pong: function() {
+      return false;
+  },
+
   close: function(reason, code) {
     if (this.readyState !== 1) return false;
     this.readyState = 3;

lib/websocket/driver/hybi.js

@@ -169,6 +169,11 @@ var instance = {
     return this.frame(message, 'ping');
   },
 
+  pong: function(message) {
+      if (this.readyState > 1) return false;
+      return this.frame(message, 'pong');
+  },
+
   close: function(reason, code) {
     reason = reason || '';
     code   = code   || this.ERRORS.normal_closure;