50 mm fan mount for Prusa v2

A straightforward port of http://www.thingiverse.com/thing:19076 to 50 mm fans. The fan has been hiked out by 12 mm. The snout is the same diameter as in the original, so the back pressure on the fan is higher. I also reduced the thickness of all panels and enlarged the holes and nut traps for a looser fit. The clearances are the same as in the original.