new 'decorated_transcript' glyph

This is the new 'decorated_transcript' glyph I was e-mailing about with Scott Cain and Lincoln Stein a month ago. Its intended use is to draw protein feature highlights specified in amino-acid coordinates (e.g., signal peptides, transmembrane regions, low-complexity regions, etc.) on top of gene models .

To see an example of how this looks like check out the image (t/data/decorated_transcript_t1.png) generated from the test script (t/decorated_transcript_t1.pl).

The 'decorated_gene' glyph, which is a new file also present in this pull request, is basically a 1:1 copy of the 'gene' glyph with 'processed_transcript' as its base class replaced by 'decorated_transcript'. This allows highlights to be drawn for multiple isoforms of the same gene.