Course, CourseImpl and Exam JavaDoc

Added JavaDoc to Exam, Course and CourseImpl.
ExamImpl JavaDoc

Added JavaDoc to the ExamImpl class.
ExamSchedule+Student JavaDoc, StudentImpl equals

Added JavaDoc to ExamSchedule and Student interfaces.
Rewrote the equals and hashCode methods in StudentImpl to only look at
id.
StudentImpl JavaDoc

Added JavaDoc to the StudentImpl class.