Add support for time zone offsets

FlySight normally names files and sets modification times using UTC. This
changeset adds a TZ_Offset configuration value, allowing the user to work
in local time.

The logged data retains the same format, e.g. "2013-09-04T02:02:39.50Z",
regardless of TZ_Offset, but this way a day's worth of skydives will be
stored in the same directory and with filesystem timestamps suitable for
correlating with video files or other captured data.
Changed USB CDC interface to compile-time option
Merge pull request #36 from crwper/make_serial_optional

Changed USB CDC interface to compile-time option
Added multiplier for chirp step
Changed atoi to strol in Config.c
Use atol instead of strtol
Merge pull request #37 from crwper/fix_config_val_truncation

Changed atoi to atol in Config.c
Cleaned up speech generation and added unit text
Removed speech text filter in UBX_Task
Merge pull request #38 from crwper/fix_chirp

Adjust chirp step to account for tone sample length
More corrections to allow for units in speech
Merge pull request #8 from willglynn/tz_offset

Add support for time zone offsets
Merge pull request #39 from crwper/clean_up_speech_generation

Clean up speech generation