Mostest Minimalistic-est Z-Probe

This idea is based on Gav's Z-Probe, but this version is mounted outside of the build area and does not require running the axis so far past the point where it would normally collide with the platform. Instead of icky glue and scrap acrylic, it uses a smaller board-mount microswitch mounted to the z-guide screws with a bit of perf board and some left-over standoffs. This version makes it possible to home all three axes without X/Y endstops (though they are recommended, and it makes terrible nois