Added rotation name arg since the username could be defined in multiple
rotations.
Renamed IOnCallService#getOnCallUser() as getOnCallResponderName().
Changed periodEndDate to endDate because it doesn't necessarily
represent that the Rotation period ended ... only that the Responder
changed.
Added config to onCallService for onCallEntryDAO.
Add constructor.
Removed getFollowingResponder methods.
Made comparable.
Fixed getOnCallProjections() and wrote some tests.