Add trace() node to convert images to vector graphic using potrace.

Prototype for a ```trace()``` module that can import images (PNG) and convert them to 2D vector graphics.

Library used is potrace: http://potrace.sourceforge.net/

Implementation is prepared to handle building with a missing library. In that case a dummy geometry is generated (just a border with the size of the bitmap) and a warning is shown in the console.

TODO:
- [ ] Decide how the API should look like, currently it's ```trace(file = "img.png", threshold = 0.5);```
- [ ] Check build on MacOS
- [ ] Check build on Windows

![openscad-potrace-1](https://cloud.githubusercontent.com/assets/1330241/5555149/3a84ef88-8c9c-11e4-8894-d344366a1f6b.png)