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. 
Can you that a bit more? I don't really see how codons help here. The exon contains stuff which *isn't* a codon (the 5' and 3' UTRs) and the offsets are from the beginning of the exon, not from the start codon of a particular transcript.