skip this die message as it doesn't work for remote VCFs
a couple of fixes, add the get_individuals method
add -f to tabix cmd line as some versions of tabix spout warnings about the .tbi being older than the .vcf.gz even when it isn't
add some extra methods and gymnastics to make fetching genotypes for a subset of individuals much much faster
Merge pull request #2 from willmclaren/variation

Add some new features and fixes for the VCF backend code in the Variation API