Adding GraphicsEx APIs and Improving Performance for Stroke.buildGeometry()

This pull request contains 4 pieces.

- Performance enhancements to Stroke.buildGeometry
- Adding the GraphicsEx API, adding natural spline curve and a post processing system for thickness, color and alpha
- Opening up classes in the Graphics API for overriding, i.e moving privates to protecteds. This also moved StrokeVertex to a public class rather than a private class in Stroke.as
- A new Example code for how to use the GraphicsEX API.

Unfortunately, I had already performed the last 3 when I did the first one, the performance boost, otherwise it might have been smarter to first submit the performance boost as a single pull request first.

If you decline this big request (I am still not sure how GitHub works fully) , I think the performance boost thing could still be valid to deliver as a separate check in. However, I might have to do some git magic in order to achieve that, and I'm not skilled enough to solve that at this point. 

Thanks

/IonSwitz