Added Contour plots with a test.

Here's my contour plot code. The bulk is the algorithm which actually generates the contours - the interface is relatively simple. I'll be open to improving it on request.

One outstanding issue is still that without any kind of line smoothing, it's hard to prevent the contours from having a blocky appearance. Over time this has bothered me less though, because the fact that standard contour plotting algorithms do apply line smoothing without informing the user is a bit disengenuous.

Anyway, maybe one day we'll get line smoothing into Chart.

![2D Sin Wave](https://cloud.githubusercontent.com/assets/2581658/4088330/ea7e6516-2f5b-11e4-8df7-15aa5de814dd.png)