Update IntSet.java

Operations working and the logic should be fine (checked with the post conditions). Messed with the array thing though, something to do with boundaries and probably how we're working with them! (capacity, the methods, array and vectors) If you see the thing clearly and can fix it, super! Otherwise tomorrow I'll have a more careful look at this ^^.
Merge pull request #1 from Talito/patch-1

Update IntSet.java