RFC: Manual Bed levelling

Levelling the bed in software is a great approach, since it makes the mechanics much more rigid. Rather than build an automatic probe, I tried measuring the bed level manually by jogging the head down in Z until it just drags a piece of paper, then entering the offset at three points in gcode.

I'm happy to tidy this patch up to get it merged, but I wanted to get this out for everyone's comments early.

This patch adds three new commands: one to set the bed levelling equation manually, one to clear it and one to position the head in X and Y for probing.