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