Add option to kill a worker from the command line

I've been using `ost-bin` for a while already and it's very useful, though the only thing that I've found missing was the possibility to kill a worker in a similar way they are started.

This pull request adds a `-K` option to kill a worker, so `ost -d worker` initialize and daemonize a worker, and `ost -K worker` sends `TERM` to the worker's PID.