Export KML with absolute altitudes

The default KML altitude mode is clampToGround. Given that most
FlySight users are interested in altitude, it seems more appropriate to
generate KML files with absolute altitudes displayed.
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.
Add Dir_format config option, supporting YYYYMMDD naming convention