Modified behavior S parameter of G1 and M3/M5 to behave more like feedrates

Implements the following changes:
*  G1 does not have a default intensity.  Instead will use last set intensity if S parameter isn't set.
*  M3 does not have a default intensity.  Instead will use last set intensity if S parameter isn't set.
*  M5 sets intensity to 0 as well as turning off laser.