使用方法：
设置 cookie：
$.cookie('the_cookie', 'the_value');注：如果
$.cookie 没有第三个参数，那么当浏览器关闭时，该 cookie 将会自动删除。设置一个有效期为 7 天的 cookie：
$.cookie('the_cookie', 'the_value', {expires: 7});注：
$.cookie 第三个参数是一个对象，除了可以设置有效期（
expires: 7），还可以设置有效路径（
path: '/'）、有效域（
domain: 'jquery.com'）及安全性（
secure: true）。读取 cookie：
$.cookie('the_cookie');注：如果没有该 cookie，返回 null。
删除 cookie：
$.cookie('the_cookie', null);我们只需要给需要删除的 cookie 设置为 null，就可以删除该 cookie。
最后附上源代码：
/**
* Cookie plugin
*
* Copyright (c) 2006 Klaus Hartl (stilbuero.de)
* Dual licensed under the MIT and GPL licenses:
* http://www.opensource.org/licenses/mit-license.php
* http://www.gnu.org/licenses/gpl.html
*
*//**
* Create a cookie with the given name and value and other optional parameters.
*
* @example $.cookie('the_cookie', 'the_value');
* @desc Set the value of a cookie.
* @example $.cookie('the_cookie', 'the_value', { expires: 7, path: '/', domain: 'jquery.com', secure: true });
* @desc Create a cookie with all available options.
* @example $.cookie('the_cookie', 'the_value');
* @desc Create a session cookie.
* @example $.cookie('the_cookie', null);
* @desc Delete a cookie by passing null as value. Keep in mind that you have to use the same path and domain
* used when the cookie was set.
*
* @param String name The name of the cookie.
* @param String value The value of the cookie.
* @param Object options An object literal containing key/value pairs to provide optional cookie attributes.
* @option Number|Date expires Either an integer specifying the expiration date from now on in days or a Date object.
* If a negative value is specified (e.g. a date in the past), the cookie will be deleted.
* If set to null or omitted, the cookie will be a session cookie and will not be retained
* when the the browser exits.
* @option String path The value of the path atribute of the cookie (default: path of page that created the cookie).
