Adding optional image fill color for background ellipse fill.

I had a need for this in my project so I added it in as optional functionality. I had some transparent-background PNGs as user avatars that needed to go into `AGMedallionView`, but with a background color that didn't expand beyond the circular border. It's all yours, if you'd like it in your project!

(And thanks for your work on this library; it's a slick one!)