reprapdiscount RAMPS screen mount

I wanted something minimal to mount the [reprapdiscount](http://www.reprapdiscount.com/electronics/19-smart-lcd-controller.html) on my prusa mendel. This uses a rehash of the `sanguinololu-holder.scad` file from the prusa mendel source. Print 2, clip onto your machine (backs first), then attach the screen.

Leaves the SD card slot accessible for auto printing.

I got the pictured control knob from here: http://www.thingiverse.com/thing:35438