Add example GTF file to the example volvox browser. The gtf is itself converted from the volvox.gff3 with the Cufflinks gffread utility. See pull #453
Remove the requirement for the gene_id and transcript_id to be set, because they can become undefined and produce errors. I read one GTF file specification that says both gene_id and transcript_id are required, but I have seen instances where gene_id is not set if there is no top level gene features.
Add the transcript_id labels to volvox.gtf.conf