WIP: new example with real-time digital signals

Not ready yet. To do:

* [x] ncols and nrows parameter
* [x] GLSL-based clipping
* [x] separate x and y in the VBO to divide by 2 the amount of data transferred (compute x coordinate in the vertex shader)

![signals](https://cloud.githubusercontent.com/assets/1942359/3778623/bd99a57e-1976-11e4-9284-f3d90e265002.gif)

Close #135.
FYI you can now do `color='white'`