Optimize dice multiplication

The optimization inspired by
http://hackage.haskell.org/package/probability-0.2.4/docs/src/Numeric-Probability-Distribution.html
Make the Coop testing setup a bit more varied
Remove and reduce old thread delays
Improve the documentation of Main.hs
Get rid of printf
Make the attacker more obvious in the combat animation
Announce last hero death before, not after game over screens
Report inventory info about picked up item only for the leader
Fix removal of server savefile in case of unusable client savefile
Don't limit inventory size
Split combat animations in attack and defense phases
Let AI ignore already dying foes
Refactor inventory item choice code
Define inventory menu keys and actions in a data structure