Added GPS date logging and logfile timestamps

- Logfile by default now contains field "Date", which is updated with
  the GPS date. This is an integer value in the format YYYYMMDD.
- Files written to the SD card are now given timestamps, based on an
  emulated RTC. RTC is set to GPS date and time if possible; otherwise
  it is initialized to 1 January 2000, 11:55:00, as per rtc.c. RTC is
  incremented every second by vTaskIncrementTick(). (There may be some
  drifting.)