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. 
1) This totally doesn't need to exist yet, it was an easy problem to pick off while stuck on other things. 

2) The use-case is that we're going to, in the near future, get mouse mutation & epitope data. Since I want this library to be usable in an exploratory fashion, it should be possible to say EnsemblRelease(75, species="mouse"). However, having a hard mapping of "mouse"->"mus_musculus" (and not allowing the latter) feels wrong, since people have to keep track of your name remappings. So, I want to support both the common name, and the binomial/latin name.