Log Message:
Changes 0.11:
* suppress warning in perl < 5.10

Changes 0.10:
* support new style: $pm->start(sub { ... })
* collect zombie processes without any delay when spawn_interval is nonzero
* do not sleep spawn_interval seconds when a worker process exits non-zero and
  if err_repawn_interval < spawn_interval

Changes 0.09:
* support for slow startup (with the new spawn_interval parameter) and slow
  shutdown (by passing arrayrefs as values of the trap_signals hashref)