added filament_diameter config setting to Extruder to make volumetric extrusion easier to enable
if filament_diameter is not set or is 0 (or less) then the steps_per_millimeter will be left unaltered
removed unnecessary members from Extruder
added M200 command to set volumetric/linear extrusion mode
Merge pull request #414 from logxen/volumetric

Volumetric extrusion