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 seems that my using the phrase "# of codons" hasn't been very clear! Anyhoo, yeah, that's all I was saying. It was just a nit point about readability, without knowing what the better option(s) are.