Collect helicopter, get 5 iron

[lightonflux]

Changed to the newest version:

commit 10f1433

and now when i dig a helicopter it will not work, or it will add 5 iron bars to my inventory instead of a helicopter.

[SokolovPavel]

I added HP system, so now you can only destroy your heli and then get 5 iron bars and mese crystal. Heli has 30 HP.
May be adding ability to pick up heli only to owner is good.