Added thermal limiting to protect motor.
New config.h parameters:
thermal_limit sets the maiximum continuous thermal load for the motor
thermal_safe_speed is the speed above which motor is thermally safe

For more details see http://www.pedelecforum.de/forum/showthread.php?19490-FC-Begrenzung-der-thermischen-Leistung-quot-Motorstrombegrenzung-quot