Support for xmas-b hardware revision

This commit incorporates preliminary support for the xmas-b hardware
revision.
Added software temperature compensated timekeeping

Communication with the DS18B20 temperature sensor seems to work
reliably now and is used for software-based temperature compensated
timekeeping as described in the thread:

http://forums.adafruit.com/viewtopic.php?f=41&t=43998

The code seems to work, but has not been well tested.
Settings-save bugfix (minor)

The per-digit brightness settings were not being saved to EEPROM, so
if the clock undergoes a hard reset (no external or battery power),
the per-digit brightness settings would be reset to the defaults.

This has now been fixed.
Minor menu text change

When setting the min and max volume for the progressive alarm, the
"v min XX" and "v max XX" screens now read "v. min XX" and
"v. max XX", where XX is the set volume.
Merge branch 'xmas-b'

The xmas-b branch included experimental support for the xmas-b (REV B)
redesign of the Ice Tube Clock:

  http://forums.adafruit.com/viewtopic.php?f=41&t=43836

The 'xmas-b' branch also introduced the option of using software-based
temperature compensated timekeeping with a DS18B20 temperature sensor:

  http://forums.adafruit.com/viewtopic.php?f=41&t=43998
Increased button debounce time

The button debounce time is now 30 milliseconds instead of 20
milliseconds.  This minor change improves recognition of multi-button
presses, currently used in the per-digit brightness controls.