Allow log2m*regwidth to be small. Fix #2.

- Made paramter-min/max constants public.
- Added random elements to HllSerializationTest.
- Allow (de)serializers to work with small register values.
- Fix a bug in BitVector#registerIterator() when there was too much
  leftover space at the end of a word.
Version bump to 1.2.1.
Add new HLL constructor. Bump to 1.3.0. Fixes #3

- Add an HLL(log2m, regwidth) constructor that defaults to automatic
  EXPLICIT settings and using SPARSE.