Added new time display formats

This commit adds support for four new time display formats, making a
total of nine.  The new formats are rolling colons ("HH:MM:SS"),
fractions of a second ("HH.MM.SS.FF"), or with single letter
indicators for AM and PM ("HH.MM.SS.P " or "HH MM A ").

The rolling colon display based on Russell Walker's rolling colons:
    http://forums.adafruit.com/viewtopic.php?f=41&t=44545

And the fractional second display is an idea taken from DigiSage's
firmware:
    http://forums.adafruit.com/viewtopic.php?f=41&t=13225