Soft kills to allow child processes to catch signals and clean up.

includes pull request https://github.com/Jimdo/periodicnoise/pull/34 so rebase after merge of https://github.com/Jimdo/periodicnoise/pull/34 is highly recommended.

closes https://github.com/Jimdo/periodicnoise/issues/32

@mlafeldt PTAL
@Luzifer and @3oris FYI