Merge pull request #476 from wolfmanjm/upstreamedge

Add M221 extruder flow rate multiplier closes #448
add ON_HALT event
turn off heaters and motors in on_halt()
call ON_HALT when a limit switch is hit
Merge pull request #477 from wolfmanjm/update/add-on-halt

add ON_HALT event
enable adjust z function when restoring three point leveling plane