Sprites again

This is another attempt at sprites, it's based on #89, but I went ahead and rebased on master to get the whole scss_refactor and the django_pyscss thing worked out.  When I rebased, all of the sprites stopped showing up.  I think something broke in pyScss 1.2.0, because I remember that the sprites were showing up before.  Anyway, I switched from using the `sprite` function to using the `sprite-url` and `sprite-position` functions because those actually work.  The `sprite` function only outputs positions of `0px 0px` which is definitely broken.  When I got the sprites displaying, they were all wacky, so I messed around with the CSS and found a way to output them perfectly (and @justinstollsteimer said it was kosher).

I checked it on Chrome and Firefox.