Add Python/C++ boilerplate for HyperLogLog counter.

 - Add a new sandbox script for initial testing.
 - Update Makefiles.
Basic implementation ready, still debugging.

 - Change signature: pass acceptable error rate to create HLL counter.
 - Implement HLL counter based on svpcom/hyperloglog Python version.
 - Use khmer::_hash function from ktable.
 - In hll_count use a collections.Counter to check results.