Allow stats commands to be used in PM. (issue #299)

- Added an optional 'channel' parameter to all stats commands to allow
  usage in PM
- Affects TriviaTime.day(), TriviaTime.week(), TriviaTime.month(),
  TriviaTime.year(), TriviaTime.me(), TriviaTime.stats()
- Minor editing of message output for those commands
- Edited some command documentation