Add SD-Card change and SD-Card init. for reprap electronics without
SD-Chard change detect pin.

[default off for now]
syntax: M600  X[pos] Y[pos] Z[relative lift] E[initial retract] L[later retract distance for removal]

if enabled, after a M600, the printer will retract by E, lift by Z, move to XY, retract even more filament.
Oh, and it will display "remove filament" and beep like crazy.

You are then supposed to insert a new filament (other color, e.g.) and click the display to continue.
After having the nozzle cleaned manually, aided by the disabled e-steppers.
After clicking, the printer will then go back the whole shebang, and continue printing with a fancy new color.

Always set the FAN PWM value in check_axes_activity() and always use
the value of the active move if a move is active.

This reverts commit cb02c2f9d8.

The number of steps per mm often contains 4 digits before the decimal point and does not require precision greater than 0.1

The joris' char drawins are expressed in binary so to show their content's in a more visual fashion.

This is not a code modification as much as a readibility modification.

The cli(); at the start of the kill() function also stops the internal
arduino timer which stops updating of millis() which prevents the
display of the "KILLED." message.
The new function updates the display directly without checking any
timers.

- related bug reports: Issue #201, Issue #213

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.

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

M303 Starts autotune. Wait till the Kp Ki and Kd constants are printed.
Put these values in Configuration.h

The percent complete now lines up with the temperatures and Z-axis position figures, nicer to read and nicer to look at IMHO.