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.