Option to invert PWM duty cycle. 

This pull request fixes issue #69, and adds an extra config variable laser.pwm.invert (default 0).  when enabled this inverts the laser power.

For now this is only actually applied in the g-code interpreter, and works by setting the power to 10000-specified power.

It should probably actually be done in the part that sets the PWM frequency, but I cant find it :/