Add a bit of responsiveness by recalculating values on resize

Not very elegant, but should force flipsnaps to recalculate all values on-the-fly when the page is being resized, providing you've used % values in your CSS for your slider containers and children. For example, this CSS is recommended:
```
#slider-mask { width: 100%; overflow: hidden; }

#slider-mask > #slider { width: 500%; } // For 5 slides

#slider-mask > #slider > .slides { width: 100%; }
```
This only works for the transform3d method in the master code as of now.