Added flag for trigger polarity and sample frequency

 * the -P flag was added to configure triggering on
   the positive edge of a slope.
 * You can change the sample frequency with the -F flag,
   but make sure to check the actual sample frequency
   via -v, because it can only be set quite coarsly on
   the eval board hardware.
Fixed errors in TextStream format

 * Date formating in the header was ugly
 * No newline after record time stamp, resulting
   in first bin to be dropped
 * Changed unit of measurement from ns to us in
   record timestamp