Implement --grace-time parameter

In order to kill more softly and avoid errors on sensitive children,
implement kill with SIGTERM --grace-time before --timeout.
explain and check --grace-time

In order to better explain what --grace-time vs. --timeout means,
change option description.

Also check for sane grace time value.
Explain life cycle of wrapped commands and pn

In order to clearly communicate the relation between all the various
mechanisms of periodicnoise, add a short section in the man page for it.
fixup trailing spaces
Updateb man-page for --grace-time
better wordings

thx to @mlafeldt
Merge pull request #35 from Jimdo/soft_kills

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