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

Not yet ready for merging. 

Preliminary support for gene models from GTF files.
We might be mixing up "how is this modeled _in the GTF/GFF formatted files_" and "how should this be modeled, logically."  I think this is an appropriate place to suggest that I've got the modeling wrong here, and we should be representing these things differently than they're handled in the files.  :-)