Added the ability to add several strokes and shadows

Hi

I needed double stroke for one of my project. You'll find how I dit it.
If you think my PR is relevant and ok to be integrated in your project, please let me know. I will adjust my modifications for not breaking the old properties + update the ReadMe

![capture decran du simulateur ios 2 juin 2014 11 37 52](https://cloud.githubusercontent.com/assets/968830/3149609/e96315b6-ea6c-11e3-9caa-6527bb1ea40a.png)