Add canny edges filter

Hi! I am going to MozFest together with @forresto, @automata and maybe more of you :), so I am planning to create some filters to work on the Turtle Power installation/workshop.

This one is a canny edge detector from JSFeat, and the image below shows it working on Flowhub.

![cannyedges](https://cloud.githubusercontent.com/assets/289130/4436142/c2af90f8-4768-11e4-8f31-e967daba3523.png)

Also, I wanted to create a test, but I ended up with a problem on node-canvas (https://github.com/Automattic/node-canvas/issues/476) so I committed only a simple version of the test, but the full version is on https://gist.github.com/GabiThume/335eabfc5fd440242b89 for later.