Add support for an atmega32u4 on a Teensy 2.0 carrier board.

The atmega32u4 timer 4 registers differ from other AVRs, requiring
changes in heater.c and mendel.c The teensy/atmega32u4 uses LUFA USB,
but does not have 'usb' in the name, requiring changes in the
makefile PID values read from a blank EEPROM are set to default
rather than using zeros.
More enablement of ATmega32U chip.

- ATmega32U timer 4 register handling differs from other AVRs:
  Change heater.c and mendel.c.
- LUFA USB recognition expanded in Makefile.
- Add section for __AVR_ATmega32U4__ in arduino.h.
analog.c: put in a few lessons learned.

No functional change.
analog.c: trim down analog values storage.

Saves 10 bytes RAM and 16 bytes binary size on an 1284P (8 ADC
channels) running two sensors. Should also be a bit faster, as one
loop runs fewer iterations.

Also, checking for NUM_TEMP_SENSORS was a mistake, as temp sensors
_not_ using an analog pin exist. Extreme case is, temp sensors
exist, but zero ADC channels are used.
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.
heater.c: Make BANG_BANG treat PWM-capable pins as non-PWM.
.
arduino_32U4.h & mendel.c: correct some PWMs for atmega32U4.
config.teensy.h: conform to upstream.
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
Makefile: Teensy-specific changes: use PJRC's usb_serial instead of LUFA and teensy's cli loader
config.teensy.h: test settings on a Wallace
lufa_serial/lufa_serial.h: drop Joystick.h lufa_serial/makefile: config for atmega32u4 on BOARD=TEENSY
createTemperatureLookup.py: update file links, add --vadc=, -min-adc= and output comments for thermistor R,V,resolution, and power
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
Makefile: add autodependencies for header files & default for Teensy
temp.c: back out DEBUG_USER code
temp.c: DEBUG_USER code for raw temp readings