Added addValue() method for adding a new answer value to the model.
Added setValueAt(Answer answer, int row) method for setting an answer
at the specified row index.
Modified Files:
src/java/de/fhworms/ztt/ox/examrepository/AnswerModel.java
src_test/java/unit/de/fhworms/ztt/ox/examrepository/AnswerModelTest.java
