概述 callbacks.has( callback )
返回值：Boolean
描述：确定列表是否有绑定任何回调。 如果回调作为一个参数提供，那么可以确定其是否在列表中。
示例
用 callbacks.has() 检查是否回调列表中包含一个特定的回调:
// a sample logging function to be added to a callbacks list
var foo = function( value1, value2 ){
console.log( 'Received:' + value1 + ',' + value2 );
}
// a second function which will not be added to the list
var bar = function( value1, value2 ){
console.log( 'foobar');
}
var callbacks = $.Callbacks();
// add the log method to the callbacks list
callbacks.add( foo );
// determine which callbacks are in the list
console.log( callbacks.has( foo ) ); // true
console.log( callbacks.has( bar ) ); // false