Main material feature

Hi,Dr Ji

     I have made some progress in dynamic-select-menu.

     The three level dynamic-select-menu has been completed on frontstage.

     I also did some work in app/admin/main_material.rb

     Next step,i prepare to improve the dynamic-select-menu by using json data and make it work on backstage

     Thanks!
model部分的东西都放到model里面，不能在controller和view里面出现。如果需要任何参数，在model里面定义一个函数去access 数据库里面的东西，然后expose那个函数给controller，最后由controller以respond_with等方式传出来。