add command M666 for adjusting delta printer endstop position

M666 allows you to specify offsets for towers X, Y, Z, which are applied just after homing. So no need to fiddle with screws when levelling heat-bed the old way. The setting is stored in EEPROM.

The typical use of this looks like: M666 X-0.03 Z-0.1
Which means after homing move carriage on X tower 0.03 mm down and carriage on Z tower 0.1 mm down.
The adjustment can be done only in the down direction since in the up direction we already hit the end of the tower.

It is only for DELTA.

I was asked to try to push it upstream here:
https://groups.google.com/forum/?hl=en&fromgroups#!topic/deltabot/Y6DXbxMOa40