Check for the Selected head (selected tool).

This will allow for multi extruder printing support in future pull
requests. It works fine for single extruder setups as well as the
selected head is always E0.
clean up a comment
Implement M190

Fixed Issue #33
Merge RRP upstream changes
Merge branch 'T3P3-selected_head_changes' into selected_head_changes

Conflicts:
	GCodes.cpp
Merge remote-tracking branch 'origin/duet' into selected_head_changes

Conflicts:
	Platform.h
Merge pull request #28 from T3P3/selected_head_changes

Selected head changes