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.
Is this `signum` call really needed here? `compareTo` should already return the correct value.