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. 
In general, keeping each PR focused on one thing would be pretty helpful for code reviewing, git history, etc. (Separate commits within a PR helps a bit, but separate PRs would help a lot more.) I realize, though, that's easier said than done when/if PRs take a while! Blocking is bad.