Gcoder support M82/M83 for relative E to give correct filament length

Slicer allows E to be relative while other coordinates are absolute, using M82/M83 gcode to set whether the extrusion axis is absolute or relative.

See http://reprap.org/wiki/G-code#M83:_set_extruder_to_relative_mode

This patch allows the filament length to be calculated appropriately.