Fix: Bug report 231 - Explosives timer is incorrect

Fixed bug report 231 - Explosives timer is incorrect:
http://openxcom.org/bugs/openxcom/issues/231

Vanilla-like grenades behavior:
http://ufopaedia.org/index.php?title=Grenade
> Timers are most commonly set to 0 (end of your turn) or 1 (end of alien turn)