Add non-linear scaling and outlining

I wanted to create non-linear outlines for bezier-curves.

![tmp-bezier](https://cloud.githubusercontent.com/assets/39374/4736721/29a8e122-59ef-11e4-933d-c3aeea634d3f.png)

I dug into the code and found that it was surprisingly easy to achieve something visually satisfying, although I'm not entirely sure that it is mathematically correct.
I hope you find it worth merging.
it makes more sense to reassign d1 and d3, so that you can simply call segment.scale(d1,d3) here. If d3 is undefined, that's just going to call the scale/1 function and we don't need the conditional