Allow heaters to stay full On when PWM=127

In previous version, even with PWM = 127, the system turns the FET off
and then on in the next cycle. This bevavior may increase the FET heat
dissipation.
It was fixed keeping the FET always On when PWM=127.
Z Axis Safe Homing when using Z Probe

Recommended for those who are using the Z Probe for Z Homing (as
Z-Endstop)

This feature has two changes:

1) Allow user to choose where the Z Probe will touch the bed when homing
all axis together (G28) by setting below defines:

Z_SAFE_HOMING_X_POINT
Z_SAFE_HOMING_Y_POINT

2) Prevents the user to perform Z Axis Homing when the Z Probe is
outsite bed.
Bed Heater monitoring in Controller Fan

In some cases the Bed Heater FET heats up more then stepper drivers, so
this change add the bed monitoring to the controller fan. As soon as the
bed heater is turned on, the controller fan will run as well.
add readme
Merge remote-tracking branch 'upstream/Marlin_v1' into Marlin_v1