1. fix a bug in hashbits.cc of 'Division or modulo by zero' described by  https://github.com/ged-lab/khmer/issues/182
 2. add test code for count-overlap.py
 3. fix a bug in count-overlap.py (a character was removed accidently some time)
 4. count-overlap.py needs hashbits table generated by load-graph.py. This information is added to the "usage:" line.