Add support for -implied_utrs and -adjust_exons to the gene glyph.

This patch adds support for the -implied_utrs option to the gene glyph, addressing issue https://github.com/GMOD/Bio-Graphics/issues/16. This change presumably adds support for -adjust_exons, though I haven't explicitly tested this.

An example of gene models from the Medicago truncatula 4.0 genome annotation rendered in GBrowse, before the patch:
![before](https://cloud.githubusercontent.com/assets/1800812/3193360/bcea653e-ecf1-11e3-8d49-57d34792f925.png)

After the patch:
![after](https://cloud.githubusercontent.com/assets/1800812/3193362/c53bfe32-ecf1-11e3-8432-2ff196324275.png)