Implement reverse iterator

For our use-case, we always want to traverse the set in descending sorted order. Adding a getReverseIntIterator eases this operation.

I've also added guava as a test-only dependency to save some code; if this is a problem I'm happy to restore the earlier code + implement some form of Lists.reverse.
Incorporating feedback

The #nextSetBit and #prevSetBit methods on both BitmapContainers now throw exceptions when passed out-of-bounds arguments, and various iterators have been updated to reflect that fact.
Merge pull request #13 from sethp-jive/feature/reverse-int-iterator

Implement reverse iterator