Centos init

This PR brings what I hope to be our CentOS celeryd init script going forward.

This frankenscript includes:
 - parts of Andrew McFague's original "working centos init script" for celery 2.5
 - parts of Ask's generic init script for celery 3.0
 - reliance on centos's built-in 'status' and 'killproc' commands which are super nice
 - support for multi-node multi-pid celery
 - standard centos service-init output
 - working exit status codes (should come in handy with those release_ais scripts)