Added InvalidMeetingException tests

Added two new tests to confirm an invalid meeting exception is thrown, if a meeting is constructed without at least one contact.
Corrected some typos in the new tests.

Added two new tests to confirm an invalid meeting exception is thrown, if a meeting is constructed without at least one contact. Ensure the current MeetingImpl class does not pass new tests.
Declared new exceptions thrown

Declared new exceptions thrown by constructor & test methods.
Added InvalidMeetingExceptions

MeetingImpl now throws an InvalidMeetingException if a new MeetingImpl object is constucted without at least one contact.
Created FutureMettingImpl

Created a simple class for FutureMeetingImpl with a constructor that calls it's super.
Created tests for the added getNotes() method in the PastMeeting interface
Corrected some typos
Retitled the ContactTest class to ContactImplTest for uniformity.
Wrote PastMeetingImpl class to pass all tests in the PastMeetingImplTest class.

Created and passes all tests.