add "wait_all_children_with_timeout" method.
add test and doc for wait_all_children_with_timeout.
delete wrong comments
clarify the meaning of $ppid
use after_fork hook instead of tempfile
eliminate busy loop
[refactor] implement timeout as an optional behavior of `$pm->wait_all_children`
Merge branch 'pr/10' (fixes #10)
releng for 0.16
Revert "use after_fork hook instead of tempfile"; the approach before the revert had a race condition between the workers.

This reverts commit ce1dd76ffc5c9e1a4b9235f25174a381e1effd34.

Conflicts:

	t/06-wait-all-children-with-timeout.t
alter the test to prove that current $pm->wait_all_children is not accepting child exits eagerly
switch to alarm() + eval pattern for implementing `$pm->wait_all_children` with timeout; the use of alarm() is justified by the fact that the module is already modifying other signal handlers
Merge branch 'kazuho/issues/fix-wait-with-timeout'
releng for 0.17