Add setNickname docs

lib/Client/Client.js

@@ -755,6 +755,11 @@ var Client = (function (_EventEmitter) {
 	Client.prototype.setNickname = function setNickname(server, nick, user) {
 		var callback = arguments.length <= 3 || arguments[3] === undefined ? function () /*err, {}*/{} : arguments[3];
 
+		if (typeof user === "function") {
+			// user is the callback
+			callback = user;
+			user = null;
+		}
 		if (!user) {
 			user = this.user;
 		}