Add maximum temperature soft and hard safety limits

[clothbot]

I had a mild scare while playing with skeinforge 31 settings where I accidentally set one of the base layer temperatures to 1900C instead of 190C.

Please add extruder firmware-managed configurable soft temperature limits with a hard stop at maxReading-1:

• Set the soft temperature limit for ABS to 240C; soft temperature for PLA might be 220C.
• If the maximum possible temperature reading is 255C, set the hard stop for 254C to prevent thermal runaway.
• If future electronics (e.g. Gen 4 with thermocouple sensing) can go significantly higher, set the hard stop for temperatures below where the extruder components begins to fail. For example, PTFE properties begin to degrade at 260C (http://en.wikipedia.org/wiki/Polytetrafluoroethylene), so set the hard limit for a MK5 extruder to 259C.

Throw warnings to the console when the soft temperature limits are crossed and the heater is on (when it's off the temperature is likely just PID-related overshoot), or when the absolute maxReading-1 limit is crossed (ever time).

[cibomahto]

I've implemented a hard stop in this commit:
4e3f299

It shuts the heater off if the temperature goes above 280 degrees (which is reportedly where the Kapton tape starts breaking down).

ReplicatorG and the firmware currently don't know what kind of plastic is being used (I think the only thing with that information is the skeinforge profile), so the soft limit suggestion might better be handled there?