+ unit test & bugfix
+ perf test and give a good measure how BitSet perform in terms of #cardinality & #nextSetBit.
+ #cardinality over a 1 million sized BitSet is 3ms
+ #nextSetBit over a 1 million sized BitSet is 2.5micros/op
+ it's not as fast as java.util.BitSet, but much better than existing ones.