Added ItemStackComparators

Alternative implementation of: https://github.com/SpongePowered/SpongeAPI/pull/267

I used @Kobata 's pull request and modified it to match @ryantheleach 's and mine proposal.

This PR uses smaller `Comparator`s, that can be used more freely as well as merged comparators.
The signum is needed nowhere because compare can return -512 or 421 as well it does not matter at all.
(String uses compare each character using c1 != c2 and returns c1 - c2)
I'll remove it everywhere.