Added support for grading without a GUI.

GradingManager renamed to AGUIGradingManager to allow the creation of a
GradingManager interface.
AHeadlessGradingManager added for GUI-less use.
Added 3 new properties for use with AHeadlessGradingManager:
1) grader.headless.path: the path for grading
2) grader.headless.start: first onyen to grade
3) grader.headless.end: last onyen to grade