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. 
It's a really simple sanity check that's unrelated to any particular reference. I so far have been using the latest release for each commonly used reference (b36, b37, b38). In this case, since the reference doesn't matter, I feel like I should either do it for *all* releases (which would take forever) or just a random one to make sure I haven't mis-parsed the IDs.