Implement Proper JUnit Test Cases for GoogleCalendarManager

The test case, testAddRetrieve, tests GoogleCalendarManager's ability to
perform two important functions of CRUD - Create and Delete.

The updated JUnit test cases are also able to:
- Show explicitly the exceptions which occur and the exception messages.
	- When the user is offline, the UnknownHostException can be caught
	  by the tester, and the message is shown.
	- By using an alternative test method (commented out by default),
	  an assertion can be caught and its message displayed.