filled background

@drsnyder 
Adding support for the ?fill=<color> query parameter, which supports [ImageMagick colors](http://www.imagemagick.org/script/color.php), including "transparent", which will convert the image to png format.

Note that this was branched from https://github.com/Wikia/vignette/pull/11, since that also touched route code and I wanted to avoid conflicts.