概述 callbacks.add( callbacks )
返回值：Callbacks
描述：回调列表中添加一个回调或回调的集合。
此方法返回绑定它的那个回调对象(this).
示例
使用 callbacks.add() 添加新的回调到回调列表：
// a sample logging function to be added to a callbacks list
var foo = function( value ) {
console.log( "foo: " + value );
};
// another function to also be added to the list
var bar = function( value ){
console.log( "bar: " + value );
};
var callbacks = $.Callbacks();
// add the function "foo" to the list
callbacks.add( foo );
// fire the items on the list
callbacks.fire( "hello" );
// outputs: "foo: hello"
// add the function "bar" to the list
callbacks.add( bar );
// fire the items on the list again
callbacks.fire( "world" );
// outputs:
// "foo: world"
// "bar: world"