Support for indexed fasta files as reference sequences

Addressing issue #317

This could be implemented may different ways, I just made it so that the fasta file and index (<fasta_file>.fai) end up in the seq directory, and the fai is used to determine the offsets for loading the sequence.

There is no explicit chunk size, but this uses XHRBlob which downloads in 64kb chunks.

There is no support for compressed fasta files. This would be doable-- it would require additional preparation-- specifically bgzipping the fasta file and making a file which contains offsets to bgzf blocks.