Slider images should fill the container when transforms are available

And cleans up some magic number-ness

![screen shot 2014-07-16 at 17 37 03](https://cloud.githubusercontent.com/assets/271645/3602068/7398500e-0d07-11e4-847c-87efb568f263.png)


I think this concept/pattern is too useful just to be inlined here. Can we abstract to something which would be useful elsewhere? like a best-fit mixin or something? what do you think?