multiple changes to the mixin:
- externalized retina media query wrapper as `retina` mixin and included support for IE8 with RespondJS
- added `retina-sprite-add` mixin for easier initialization, use of multiple sprites and to use `sprite-url()` as few times as possible to improve compilation time
renamed retina-sprites
added `retina-background` mixin and changed file structure
updated demo
updated README