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

Not yet ready for merging. 

Preliminary support for gene models from GTF files.
Hmm...if they're all being serialized using the `Feature` Avro schema, then after reloading that data as an RDD, you need to map the objects so they're viewed as the appropriate type of feature (e.g., BED, GFF, etc.).  Actually, should we add a field to the Avro `Feature` schema to annotate what should be the default model for it once it's loaded?