Added a boz-sizing property

When you add a fixed-width (for example, 80%) and then try to add a padding, the actual width changes. When setting the property `box-sizing: border-box;` the padding will not affect the total width of the element. So 80% is actually 80%. More information about this property in [CSS-Tricks Box Sizing](http://css-tricks.com/box-sizing/) and the extreme use in [Paul Irish's * { Box-sizing: Border-box } FTW](http://www.paulirish.com/2012/box-sizing-border-box-ftw/)