[ADAM-327] Adding Gene, Transcript, and Exon models

Not yet ready for merging. 

Preliminary support for gene models from GTF files.
In this case, it means (I believe) the "the full spliced mRNA sequencing, including UTRs."  

But actually, maybe there should be _three_ different methods, extracting each of the (different) sequences you've described.  That's a good suggestion, I'll do that.