Functionality to gradebook

After figuring out to remove the GradeBookPanel.java and simply add all
that functionality into the button on ClassPanel, I was able to have a
gradebook (of that course) pop up and edit it.

One problem to still fix is that after saving it once, if I add a row
and click save again it will throw an error (instead of check for empty
again).
Merge pull request #11 from JeffScaturro/DESKTOP

Functionality to gradebook