put test executables in ${BUILD_DIR}/bin
change < to <= when extending reference alleles

in the AlleleMerger, when merging reference alleles from multiple
entries, we don't need to do anything when an entry is completely
contained in the current merged ref allele. We were comparing with <
rather than <=. This doesn't change output, it just keeps us from
running code like "append the empty string to the reference allele".
bugfix: handle funny indel encodings in vcf-merge

vcf-merge didn't handle indels where the padding bases were at the end
(which is allowed by the vcf spec). The outdated, somewhat complicated
logic has been replaced with something much simpler and direct (using
RawVariant to find the actual variant being described, and splicing that
in to the reference).