Marlin v1 - BUG FIXES / CORRECTIONS

Melzi support

Fixes #153 for bad call to abs()

changed encoder pos from int to long to fix problems setting x/y/z/e stepps_per_mm using the lcd menu

Marlin v1 ultimaker rc3

Made a more granular configuration possible PREVENT_LENGTHY_EXTRUDE. You can now decide to allow/disallow large length of extrusions to be executed.

Turning off the protection might be beneficial to those who want to do fast and long reversals.

Made language.h more flexible. Automatically do the right defines based on MOTHERBOARD == 7 ? "Ultimaker" : "Mendel"

This reverts commit 3682d9bd.

This is required when moving the buttons_init() function to the
lcd_init() function

Since the class "MainMenu" was used within a static variable the
initialization of the object (constructor call) was done before Arduino
library startup. It always caused a crash when using AVRStudio with
JTAG debugger (caused from calling the LCD initialization / the lot of
I/O work / the stack used during this calls). By moving the LCD_INIT
out of the constructor and using an explicit call inside of Arduino
setup() implementation immediately fixed all problems and the JTAG
debugger runs fine.

"i" runs from 0 to 4 but "add_homeing" array size is 3 only. On the
other hand the calculated value gets overwritten by either one of the
if choice.

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.