Soft PWM. (Sanguinololu can also have PID temperature control)
Interrupt save WRITE for addresses > 0x0FF

- The temperature control is pretty much complete
  (not sure what to do w/ autotemp though)
  Changed the pins assignment to clearly separate bed and extruder heaters
  and temp sensors, changed a bit how termistor tables are handled.
- The steppers control is rudimentary
  (only chanages what pins it uses depending on the active_extruder var,
   but that's enough for switching extruder in the start.gcode in the
   the profiles)
- Tested only w/ RAMPS 1.4

Found a couple of unused variables, that I commented.
Tried to solve the program memory warning message, and failed.

Add serial checkRx in stepper ISR.
Copied HardwareSerial to MarlinSerial (Needed for checkRx).

and the mechanism so that if an endstop is hit it the ISR, the steps_to_be_taken are stored, and some current_block data that will be deleted in the next move
If the normal loop() then finds such an event, the position is calculated (floats would have taken too long in the ISR) A serial message is generated.

This reverts commit 8bcdb9f5f0.