Adding sides of the DEM

![slection_009](https://f.cloud.github.com/assets/6261413/2193856/84b831f4-9883-11e3-9750-3a5709d5a487.jpeg)
Hello !
Great plugin !  It helped me for my internship to visualize easily my DEM in 3D.
I just was curious to see this visualisation with an "extruding" aspect so I created a function build_sides() to make this and the result is pretty cool. Let you see the picture above to make you an idea.

Hope you like it, and tell me if you want me to change something.