complement solutions to Exceptions in Lab 0.2.

Augmented answer to Question 2 in the following file:

	exceptions_answers.md

Besides, the complete GradeManager package has been shipped:

	src/grades/GradeManager.java
	src/grades/InvalidGradeException.java
	src/grades/LetterGrade.java

Thank you!
							Lawrence X. Amlord
							  May 31st, 2014
							  Xi'an, China