Zach's changes to; Courses, MainFrame, Openfile and Preloader

Changes:
 * Preloader; A few changes put MyCourses into an ArrayList and took off static from methods
 * Mainframe; to fill table up with gradebooks from the gradebook folder, NOTE: more things will be needed to be fixed later
 * Fixed Openfile to put information into a MyCourse object 
 * Renamed Courses class to GradebooksWindow NOTE: can be changed back if it really hurts something