Added custom capability checks. (issue #300)

- Added TriviaTime.isTriviaMod(), TriviaTime.isTriviaAdmin() and
  TriviaTime.getTriviaCapability() to allow better control over TriviaTime
  commands
- Updated TriviaTime.acceptdelete(), TriviaTime.acceptedit(),
  TriviaTime.acceptnew(), TriviaTime.add() and TriviaTime.authweb() to use
  the new capability checking methods