Replace Bio::DB::Fasta with faidx_xs

Created by: willmclaren

Benefits:

• can index bgzipped FASTA files (though note Ensembl distributes gzipped, must be converted to bgzipped before use). Human toplevel file is ~1.1g bgzipped, 40g uncompressed.
• indexes much more robust, won't tolerate corrupted/partial index like Bio::DB::Fasta
• around 10x faster retrieving sequence

Requires:

• htslib (https://github.com/samtools/htslib)
• faidx_xs (https://github.com/Ensembl/faidx_xs)
• bgzipped FASTA file

The .travis.yml install of htslib and faidx_xs is non-ideal as it avoids using "make install" by compiling the module into the home directory. Much better if you have sudo access to do "make install" properly for each.