Prusa I3 MK2 PSU cover with rear-exit NEMA-15 socket

// 2016dec05 - fiveangle@gmail.com
- fixed minor errors by MilanNXT (accidentally had degree 2-degree offset in NEMA-15 socket relocation, causing issue with interface of PSU shelf cutout + nut trap length)
- relocated NEMA socket to far left corner to prevent conflict with PSU mains crimp connections (MilanNXT version had centered, so required removing crimp lugs and twisting bare wires into PSU - see comments on my make of his design: http://www.thingiverse.com/make:272849
- re-oriented swi