Add serial_teensy.c using Teensy usb_serial.c support instead of LUFA.

Add serial_teensy.c using USB through Teensy's usb_serial.c (alternative to LUFA).
config.teensy.h: align better with config.default.h.

This updates a number of comments but shouldn't change
anything functional.
heaters: allow to disable PWM on PWM-able pins.

This is not only handy for debugging non-PWM mode, it may also
help those who run a heated bed attached to a solid state relay.

This way it was found out connecting heaters to non-PWM pins
works just fine and did so for a loooong time.

Not yet distributed to all the config.h templates.
config.teensy.h: add adjustable pwm after config.default.h
config.teensy.h: test settings on a Wallace
debug.h and temp.h: define a DEBUG_USER code and apply it to temp sensing
temp.c: apply DENUG_USER it to temp sensing
temp.c: back out DEBUG_USER code
temp.c: DEBUG_USER code for raw temp readings
serial_teensy.c: deleted.
temp.c: change the DEBUG_USER... test to optimize itself away if DEBUG_USER==0
temp.c: revert to upstream, discarding debug stuff
Try cyberwizzards lookahead on teensypp
config.??.h: test for LOOKAHEAD + ACCELERATION_RAMPING and comment an unused variable
dda_lookahead.h: Allow non-LOOKAHEAD compilation
config.teensy.h: disable DEBUG
config.[SOME].h: #ifdef LOOKAHEAD
dda.*: drop lead-axis tecking
dda.c: Replace mistakenly elided bracket
Port to Teensy3.0 (ARM Cortex-m4 mk20dx128)