event: make setMaxListeners() return this

setMaxListeners was the only EventEmitter method that returned
undefined, so could not be chained.

doc/api/events.markdown

@@ -80,6 +80,7 @@ added for a particular event. This is a useful default which helps finding
 memory leaks. Obviously not all Emitters should be limited to 10. This function
 allows that to be increased. Set to zero for unlimited.
 
+Returns emitter, so calls can be chained.
 
 ### EventEmitter.defaultMaxListeners
 

lib/events.js

@@ -49,6 +49,7 @@ EventEmitter.prototype.setMaxListeners = function(n) {
   if (typeof n !== 'number' || n < 0)
     throw TypeError('n must be a positive number');
   this._maxListeners = n;
+  return this;
 };
 
 EventEmitter.prototype.emit = function(type) {