Using with Node

As stated at #95, tracking.js is easily used within a node application by taking advantage of nodeunit's sandbox. 

In this PR i add an example of how simple would be to create a `index.js` for node applications using the sandbox and a concrete example - just like `face_hello_world.html` but using [node-canvas](https://github.com/Automattic/node-canvas)  and writing the result (rects around the faces found) to a *.png* file.

There's also a new test (`./tests/Functional.js`) which i used so that i could test what i was doing for #92 in a fast way. It uses a canvas and verifies if the expected number of faces in an image is actually found.

In #95 i raised the question about creating the `./index.js` with nodeunit's sandbox. 

What do you think?