Add NonBlockingHashSetLong and fix bug.

Add NonBlockingHashSetLong (with additional improvements to avoid
auto-boxing). Fix a bug in retainAll() in NonBlockingSetInt (wasn't
properly returning the 'modified' value. Added a test to verify the
behavior of retainAll().
Merge pull request #4 from pkwarren/non_blocking_hash_set_long

Add NonBlockingHashSetLong and fix bug.