add option to specify bam-readcount version and also set its default to ...

...0.7
Yeah - these two modules contain a lot of code duplication and could be refactored, but given that they will be probably phased out in favor of the variant-reporting subsystem over the next few months, I'm going to suggest that we just fix this quickly and move on.

@dlarson - I'm testing my changes now - you can just crib off them if you wait an hour so until I push.