Added support for a border with a given color and width

Hi Oliver

Thanks for putting this popover together. I needed a border for it, so here it is.     
The change is straightforward, basically I use the UIBezierPath stroke after all other elements are drawn.     

I have created three constants to show the border (default to NO). set the color and set the width.

Here is what it looks like:

![popoverWithBorder](https://f.cloud.github.com/assets/1455711/267581/90aeb844-8ea1-11e2-8b32-f7dcdb32f17e.png)

Cheers

David