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

Not yet ready for merging. 

Preliminary support for gene models from GTF files.
I guess I'm not sure what the meaning of `extractSequence` should be for a `Transcript`.  Does that mean the fully unspliced RNA molecule?  Or does it mean the full spliced mRNA sequence, including the UTRs?  Or does it mean just the protein coding sequence?