Added the impenetrable unsets

Fully done by @atomicint
Added a check for the amount

Alot of times when writing Skills, amount checks are needed. I kept the
original boolean and made a new one that has the amount parameter. Also,
made another one for the Item class.
Merge pull request #8 from Arham4/master

Impenetrable unsets & contains(x) methods by Arham4