Revert last change on new logfile structure and added nulling of buffer.

The reason for this is, because the last change in the file structure is buggy, it was only written the message length of the buffer to the file and missed to add the length for the timestamp.
For fixing this problem it would be possible to insert something like

len += sizeof(quint64);

after:

int len = mavlink_msg_to_send_buffer(buf+sizeof(quint64), &message);

But I think a consistent file structure has more adavatages for a logfile over a more compressed structure.
E.g. it would be necessary to go through all packages to proceed a jump during a replay.
Additional the LogPlayer was not updated during the last 7 month since the logfile structure was changed (even it was buggy)...

If filesize is really a problem I would recommend to change the logfile handling in a way of writting a temporary consistend
logfile and compress it afterwards (QCompress) like done by many programs
Merge pull request #244 from cehberlin/master

Fix/Revert problematic logfile structure changes
Merge branch 'master' of github.com:mavlink/qgroundcontrol into sensor_hil
Merge branch 'master' of github.com:mavlink/qgroundcontrol into sensor_hil
Better differentiation between estimator position and raw GPS
silenced settings load
Build fix
Icon fixes, HSI shows satellites correctly rotated
Ask mavlink app to start on PX4 boards if connected via NSH
Store directory where the last log file was stored
and reuse it when loading or saving new log files
Fixed typo
Merge pull request #246 from cehberlin/master

Save and store last log file directory
Merge branch 'master' of github.com:mavlink/qgroundcontrol into sensor_hil
Fixed up logplayer button
Merge branch 'master' of github.com:mavlink/qgroundcontrol into sensor_hil
Simplified link creation / connection a bit
Fixed up first version of sensor level HIL, needs more validation, but operational
Minor modifications to sim value display, not optimal yet
Merge branch 'master' of github.com:mavlink/qgroundcontrol into sensor_hil
Small code/docs cleanup in XPlane sim link