bug fix - mavutil forgot to write its timestamped log.

mavutil can create the timestamped logfile but forgot
to write anything to it. This fix is to get the log file
written.

Signed-off-by: Quy Tonthat <qtonthat@gmail.com>
Merge pull request #148 from qtonthat/write_log_bugfix

bug fix - mavutil forgot to write its timestamped log.