修改测试用例写法
添加pushStack, end方法
Merge remote-tracking branch 'upstream/dev' into dev
修改一下方法，让其以栈的形式返回新的结果集，结果集变化后可以通过end方法返回结果集。方法包括：slice, eq, first, last, map, filter, find, not, closest, add, parent, parents, prentsUntil, next, nextAll, nextUntil, prev, prevAll, prevUntil, sibilings, chidren, contents, appendTo, prependTo, after, before.
添加测试用例
Merge remote-tracking branch 'upstream/dev' into dev
添加测试用例
Merge pull request #570 from liaoxuezhi/dev

添加stack功能，当结果集发生变化时，可以通过end方法找回来