Added a timer for joining the game. (issue #20)

- Usage: set a non-zero value for var.JOIN_TIMER_LIMIT in seconds
- Added kill_join() to handle game cancellations
- Join timers set on first join and canceled if everybody quits or when a
  game starts
GA dies when guarding, victims name not revealed
Set JOIN_TIME_LIMIT to 3600
guardian angel can choose not to guard anyone at night
Merge pull request #39 from jacob1/master
Add missing period
Small consistency fix