Bugfix to use snp_assayed instead of deleted npg_snp_id; allow 'N' to represent a no call; correctly process null allele in converter; warn instead of crashing on absent snp/sample ID]
Squashed commit of the following:

commit aabab0d97848a2f3eeced3db69f4e328500883a9
Author: iainrb <ib5@sanger.ac.uk>
Date:   Wed Aug 27 16:55:19 2014 +0100

    Do not allow N as a substitute for 'No Call'

commit 0871586afd3c8c709dd6f380cf4615f448de0de0
Author: iainrb <ib5@sanger.ac.uk>
Date:   Wed Aug 27 16:54:56 2014 +0100

    Add no-calls to test data