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 surprised you have no comment on this! 

What do you think of having a "common" module? Is there some other way to tell all the Cache objects to use the same cache subdirectory without this silly one line file? Are there other things that should be in here?