Add beetle barrage to SVG drafts.

http://secretmaryo.org/phpBB3/viewtopic.php?f=10&t=3900
Add Timer#interrupt to the scripting interface to forcibly halt a timer.
Terminate timers on level finish. Fixes #40.

When the level’s MRuby_Interpreter instance gets destructed, it now
terminates and frees all the timers. Note that previously the cTimer
instances weren’t freed at all, causing them to lie around forever,
creating a memory leak if you play many levels with timers in a row.