several modifications about count-overlap.py script

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