Leraar Enum Added, OpdrachtTest Added, Quiz Class Added, Opdracht Updated, DatumGc Updated

New enum: Leraar.java
	- added leraars

New test class: OpdrachtTest.java
	- junit tests(complete)

New class: Quiz.java
	- empty

Opdracht.java UPDATE:
	- new parameters

DatumGC.java UPDATE:
	- equals and hashCode methods included