Changed Yeti behaviour. You must stun him (jump on him) when is under a falling stala...

Changed Yeti behaviour. You must stun him (jump on him) when is under a falling stalactite. The falling stalactite will kill him if he is stunned.
Suggested improvments:
  - have to hit Yeti with three stalactites to kill him
  - better falling stalactite behaviour--it's hard to kill the Yeti when the closest stalactite to Tux is the one that falls down

SVN-Revision: 2355