Super Ellipse Rectangle

I added support for drawing round rectangles with super ellipse formula (http://en.wikipedia.org/wiki/Superellipse) which was also used in iOS 7.

Here's a comparison (round rect on the left; super ellipse round rect on the right)
![rounded_rect_vs_super_ellipse](https://cloud.githubusercontent.com/assets/381234/2564976/0007ac02-b8ac-11e3-9f93-67578ce95990.png)


Code ported from: http://zehfernando.com/2013/drawing-super-ellipse-rounded-corners-in-actionscript-3/