Reference

Created TranscriptReferences object for downloading and accessing FASTA reference file for transcript IDs. Also, moved GTF dataframe methods from EnsemblRelease to new GTF object. 
On reflection, it seems simpler/saner to have a consistent policy for which releases we're testing. I think {54, 75, 77} seems like a good policy. If a new reference ever gets added to the mix (or we're dealing with other species or whatever), we can just modify `releases` in `test_common`.