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. 
I'm not sure if we're on the same page but just to clarify: `len(self.start_codon_offsets)` isn't counting the total number of codons but rather how many start codons that exon overlaps. 

I guess you're saying it would be easier to do something like `len(self.start_codons)` if we had a `Codon` object? I agree that would be clearer but I'm not sure there's utility to a `Codon` object, what would we put on it? The offset already contains `start`, `end`. I guess we can also add the triplet nucleotide sequence, but I can't imagine using it that way.