Modified: JavaScript file

	Name: mainpage_pic_slider.js
	Description: complete the logic and pass the test.
Merge branch 'master' of https://github.com/scutLaoYi/shengwu
Merge branch 'master' of https://github.com/scutLaoYi/shengwu
Upgrade: mainpage

	Picture slider added!
	Add a img element inside the rollDiv element.
	Add JavaScript code to make it change every three seconds.
	Make some change inside css file to let the picture fix the page.