Re-arrange layout of temperature calibration values in eeprom.
Move twi_error() from application_twi.c to twi_master.c
(twi_master_error()) so it can be used generic.
Add support for twi ds7505 on sensor 2.
Temperature sensors: merge some identical code.
Separate watchdog from application.
Merge more temperature sensors code (tmp36 and bandgap).
Major cleanup.

- created application_pwm for pwm functionality, implemented basics,
  init, interrupt handling, periodic function but no user commands
  yet
- removed generic timer0, timer1 and timer2 code because most of the
  code is not going to be used, moved timer1 code to application_pwm
- added timer1 overflow interrupt (122 Hz) for periodic functionality,
  abandoned watchdog timer for this purpose, so the watchdog timer
  can be set to any (no too smal) timeout
- application init no longer determines watchdog timeout
- added heartbeat led on 1 Hz and command led
- added "missed ticks" to enable the periodic functions to keep up when
  they're not called frequently enough
- removed PORTD0 and PORTD1 as debug leds for enc
- timeout while waiting (interrupt in sleep mode) no longer counts
  as rx error
Cleanup: removed generic watchdog code and integrated it into main.c
The code was so little there was no use having it in separate files.
Also it's only called in main.c now.
Change "unhandled periodic interrupt" stats into "max unhandled"
as the "unhandled" value is always zero at this point and less
informative.
Abandon the concept of waiting in sleep mode for a frame to
arrive, which would be announced by an interrupt from the
enc28j60. But this mechanism is so fragile, even with some
workarounds, it can't be made reliable.

So removed the sleep mode when waiting for a frame and return
immediately if there are no frames in the buffer. Then, in
main issue a sleep mode, which is then waken up from by any
interrupt (either periodic timer1 122 Hz or watchdog or frame
arrival from enc28j60) and frame arrival is checked.

This means most of the time after wake up there will be no
new frames, but at least it allows the cpu to go into  sleep
mode ~120 times per second instead of just running round in
busy mode.

The main advantage of this approach is that a newly arrived
frame is detected quickly, even when the interrupt from the
enc28j60 fails.
Change watchdog interrupt timeout to 2 seconds which should be OK.
Improved pwm test, can be checked better this way.
Better periodic tick scheduling.
Light sensor (tsl2550) support.
Makefile: Move application headers to specific object.
Makefile: add eeprom object header file dependency.
application-temperature: rename application_temp_read_init() to
application_temp_init().