Added support for M133, M134, M135, M136 & M137 so gccode from
Makerware cnverts without error. There is also a -g command line flag
and gcode_flavor option to select Makerbot/ReplicatorG interpretation
of M6, M109 and M116.
All features are adressable from the macro language.
Ditto printing should work properly.
Target pisition only calculated by the commands that use them.
Tool changes implemented according to convention.
M300 and M420 support implemented.
All requested features have now been implemented and tested. This
includes left and right filament scaling, tmperature overrides and
ditto printing. Slic3r post processing is also supported natively.