Shakedown

Attached the extruder board. Left the extruder off and added a resistor to simulate the thermistor. Loaded the minimug model and ran a build to see if it could make it through without shaking apart. The Y motor coupling came loose but everything else seemed to work reasonably well. Drilled out a pit for the Y coupling grub screw. Should keep it from slipping.

The good news is that it finished the build without a) crashing the host software, b) coming apart or c) bursting into flames.