Changed separated background-* declarations to background shortcut

Hi Twolfson,

the title of this comment says it all: I changed the sprite mixin in all style templates to output the background shortcut instead of background-image, background-position to save a couple lines of code. I chose to fork your repo, because I use it extensively in a bunch of projects via grunt-spritesmith, but as I said before I wanted to have the shortcut instead of separate declarations.

Maybe you want to merge my changes with your original json2css repo.

Best regards!
webastronaut