Reviving centos init script

This pull request resolves https://github.com/celery/celery/issues/1217 on CentOS. (Tested on CentOS 5.7)

Highlights:
 * this init script seemed to have fallen substantially behind celery 3.0 and was using celery 2.0-isms; I've updated it to play like the generic 3.0 script
 * uses centos built-ins `status` and `killproc` from `/etc/init.d/functions`, which are super-nice
 * support for multi-node / multi-pid celeryd service
 * standard color-coded centos service-init output
 * tests (!)