Introduce AvatarStatBlock class.
Convert the user interfacing classes to use the AvatarBlockStats class.
calculate derived stats on the fly instead of caching those.
Move hero_cooldown to AvatarStatBlock.
Entity has a StatBlock pointer now.

needed to fit in the subclasses.
Merge remote-tracking branch 'mainline/master'

Conflicts:
	src/GameStatePlay.cpp
Merge remote-tracking branch 'mainline/master'

Conflicts:
	src/GameStatePlay.cpp
Made StatBlock dtor virtual.
Merge pull request #378 from stefanbeller/master

Starting to cleanup the StatBlock class.