fix small bug, auto save after adding new course
allow user to change course infomation, such as name, ID etc.
a critical bug, If user fail to add new course at first time, then second time will give an exception. The program will crash then
some changes related to edit course, the critical bug
a small bug, when user enter a non-interger in worth, a error message will popup
a small bug, now adding a new category will autosave
Merge pull request #92 from LilongHuang/branch6

Found a critical bug, 3 small bugs, and added edit course feture