Fix for saveImage background color.
Fix for saveImage background color.

Followed along with:
http://www.mikechambers.com/blog/2011/01/31/setting-the-background-color-when-generating-images-from-canvas-todataurl/

Fixes #173.

jessiejea/Flotr2,alanponce/Flotr2,nicgallardo/Flotr2,beastlike/Flotr2,RobertoMalatesta/Flotr2,alanponce/Flotr2,RobertoMalatesta/Flotr2,jessiejea/Flotr2,beastlike/Flotr2,HumbleSoftware/Flotr2,HumbleSoftware/Flotr2,jessiejea/Flotr2,nicgallardo/Flotr2,alanponce/Flotr2,beastlike/Flotr2