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. 
Ah, I think this comment ended up in the wrong place. I was just saying that in `contains_start_codon`, `len(self.start_codon_offsets)` felt like a slightly confusing way to count # of codons. But I see that it's just pulling from the feature, and doing the same in `transcripts.py`. I don't have a better suggestion at the moment.