Melzi support

Fixes #153 for bad call to abs()

Added 2nd extruder temperature LCD readout and setting.

adding M120 and 121 for turning on and off the endstops only for homing

Fix to allow M120/M121

Fix compile error on boards without a bed heater.

I could not find why this problem was introduced. 
Its quite obvious, so I think its in general interest to merge.

Wrap the call to turn off the bed heater in a #if to prevent a compile error on boards that do not have a bed heater

reinitialize the lcd, if the SD card is inserted or removed. It seems that LCDs can confused by electrical discharges from time to time.
Since a removal or insert of the sd card takes a long time in firmware, just reinitializing the lcd does not hurt.
actually, it solves a problem with the UltiControlle

see https://github.com/ErikZalm/Marlin/issues/148

make should rebuild if configuration or Makefile is changed

Changed SLOWDOWN. IF this does not work ok OLD_SLOWDOWN is the old algo.

Fix for wrong systax in #ifdef.

This breaks temperature readout for pronterface!

- Fixed a typo

Fixed small bug in arcs
M105 prints now actual_temp/target_temp

Moved the PID_dT in the Ki and Kd calculation from the configuration.h to temperature.cpp