Fixed stop print LCD function on M104
Inverted state logic for forced_heating_stop variable
Changed the type of variable to bool, the name to "cancel_heatup", flags implementation and added this fix to M190 gcode too.
Merge pull request #965 from bq/forced_heating_stop

Fixed stop print LCD function on M104
Add "Thermal Runaway Protection" feature

This is a feature to protect your printer from burn up in flames if it
has a thermistor coming off place (this happened to a friend of mine
recently and motivated me writing this feature).

The issue: If a thermistor come off, it will read a lower temperature
than actual. The system will turn the heater on forever, burning up the
filament and anything
else around.

After the temperature reaches the target for the first time, this
feature will start measuring for how long the current temperature stays
below the target minus _HYSTERESIS (set_temperature -
THERMAL_RUNAWAY_PROTECTION_HYSTERESIS).

If it stays longer than _PERIOD, it means the thermistor temperature
cannot catch up with the target, so something *may be* wrong. Then, to
be on the safe side, the system will he halt.

Bear in mind the count down will just start AFTER the first time the
thermistor temperature is over the target, so you will have no problem
if your extruder heater takes 2 minutes to hit the target on heating.
Changed the declaration of the variable to Marlin_main.cpp to fix issue on commit #965
Merge pull request #989 from bq/cancel_heatup

Changed the declaration of the variable "cancel_heatup" to Marlin_main.cpp to fix issue on commit #965
Allow step rates less than 32 steps/sec

Change block initialization in planner.cpp to allow motion rates less
than 32 steps/sec.

Old behavior: planner.cpp set stepper interrupt rate to the highest step
rate among steppers. In case that rate turned out to be too slow,
stepper.cpp increased stepper interrupt rate to the minimum supported
rate, that increased motion rate.

New behavior: introduce a new macros STEPPER_MIN_TICK_RATE in stepper.h.
In case nominal_rate is too low, increase both step_event_count and
nominal_rate proportionally.
Config changes

1) Inverted non-existent endstops (ad-hoc bugfix).
2) Tried to disable them by redefining {X,Y}_{MIN,MAX}_PIN to -1, but pins.h is included after Configuration.h, so this didnt work
Merge branch 'Marlin_v1' of github.com:ErikZalm/Marlin into config-ak
Merge branch 'stepper-slow-motion' into config-ak