Damage system & place sound

2 commits that require a protocol version change:
* The new damage system that is described here: http://dev.minetest.net/Damage_system; common branch: https://github.com/PilzAdam/common/commits/damage_system
* A place field in the sound table of tools wich allows different place sounds; common branch: https://github.com/PilzAdam/common/commits/sound

Both changes are backwards compatible.