Improve 'timespans' filtering

Make all parameters to 'timespans' optional and add "parent" parameter,
returning only top-level timespans by default.

Signed-off-by: Stian Ellingsen    <stian@plaimi.net>
Signed-off-by: Alexander Berntsen <alexander@plaimi.net>
Reviewed-by:   Alexander Berntsen <alexander@plaimi.net>