Adding two new fields, 'featureType' and 'source', to ADAMRecord.

The existing implementation was parsing these into a single field, the
'trackName'.  That's fine, but we also need (for GTF/GFF files where
we'll be extracting and reconstructing gene models) a version of these
fields separated out in their original form.
Merge pull request #18 from genomebridge/gtf-fields

Adding two new fields, 'featureType' and 'source', to ADAMRecord.