Added support for a tabix backend

These additions create a basic tabix backend and support to run it from the server.py script. I designed the interface to work with both traditional VCF files and VCFs split by chromosome like the 1000 Genome release at ftp://ftp.1000genomes.ebi.ac.uk/vol1/ftp/release/20130502/ .

A tabix backend offers the benefit of quicker indexing and smaller file sizes compared with wormtables, at the expense of poorer performance.
s/index/indexed