added content box-sizing to icons

If you're using `box-sizing: border-box` on your page, sweetalert icons will show up like so:

![image](https://cloud.githubusercontent.com/assets/1147390/4516473/e805146a-4bf8-11e4-809a-43ba72749c11.png)

This PR adds explicit `box-sizing: content-box` to sweetalert icons.