Faster HLL cardinality

Math.pow(2, (-1 * X)) is the same than 1.0 / (1 << X) but much slower.

The new implementation is from 2 to 50 times faster depending on the set cardinality.
Faster HLL cardinality for small sets

Avoid to traverse the register set twice for small sets.
It does not imply any slow down for large sets.
Merge pull request #36 from cykl/faster_hll_card

Faster hll card