Increased GPS synchronization accuracy

With the GPS mod, the clock now synchronizes time to within a half
second of the GPS-reported time, as opposed to 1.5 seconds before.

The disadvantage of this change is that automatic drift correction
statistics are not collected while receiving GPS time data.  The many
corrections required to synchronize time with subsecond accuracy would
introduce too much rounding error.  But those with a GPS module should
not be too concerned with timekeeping drift correction anyway.
Merge branch 'master' into gps-exact
Enhanced filament driver options

The config.h file now includes macros to lower the frequency of the
filament drive as well as macros to run the filament under DC in both
the forward and reverse directions.
Documentation edits

Updated the acknowledgements in firmware/CREDITS and hardware/README,
added a list of features to hardware/README, and updated a link in
firmware/README.
Generalized rolling colon code

All time displays using space as a separator now support rolling
colons.  The user may now select among a simple space and four
animated styles.
GPS synchronization with autodrift improvements

With the GPS mod, the clock time will be synchronized to the GPS time
within a second.  The GPS corrections are again used to calibrate the
automatic drift correction.
Optionally flash dots-separators in time formats

The time display styles with dot-separators may be configured through
the menus to blink slowly, blink quickly, or not blink.  The previous
behavior was for all dot separators to not blink.