Add option to spawn missing workers with delay

We're running a dozen of servers with resque-pool managing resque workers, and there is one issue this pull request allows to address.

When resque workers break on startup (i.e. bad code was deployed), resque-pool goes into a loop that keeps respawning dead workers, putting the server under very heavy load. `spawn_delay` option would add a delay before respawning every missing worker and should keep CPU from redlining when resque workers fail to start.