Add Cyberwizzards lookahead. Thanks a lot, Cyberwizzard.

For now only the two new files, tweaked a bit in the #ifdef
area to let them compile without LOOKAHEAD being defined.
Make lookahead basically working.

This means, modify existing code to let the lookahead algorithms
do their work. It also means to remove some unused code in
dda_lookahead.c and reordering some code to make it work with
LOOKAHEAD undefined.
Bring LOOKAHEAD into the config templates.
usb_serial.c: eliminate type punning compiler warnings
temp.c: DEBUG_USER code for raw temp readings.
Remove M84.

Teacup handles motor on/off automatically and if your
intention is to stop the printer, M0 is appropriate (and
conforming with the NIST G-code standard).

That said, M84 is kept as a synonym for M2 to enhance compatibility
with slic3rs default end-G-code.
Rename M200 to M119 to meet Sprinter & Marlin.

No functional change.
Move the branch input-float into an attic.

This was a very interesting approach, but for the forseeable
future it's unlikely the code will replace the current one.
Introduce ATOMIC_START and ATOMIC_END.

As a sample application, use it in queue_empty().

There's also ATOMIC_BLOCK() coming with avr-libc, but this requires
a C99 compiler while Arduino IDE insists on running avr-gcc in C89 mode.
Apply ATOMIC macros in a number of other obvious places.
dda_queue.c: Simplify queue_flush().

No ATOMIC wrapping required and setTimer(0) actually restarts the timer,
which isn't what we want here.
dda_lookahead.c: use the new ATOMIC macros here as well.
switch off extruder stepper instead of burning it through
gen3 default which makes extruder actually work
power_on() for gen3 extruder board heating
remove unused and wrong code
make M106 work on gen3 hardware
fan startup threshold
option to invert heater pin
onewire ds18b20 temp sensor