use `sudo kill` for signaling when subprocesses are started with sudo

In order for this to work, add to `/etc/sudoers`:

    HUB_USER ALL=(JUPYTER_USERS) NOPASSWD:/bin/kill
use pgrep instead of ps to get sudo pid

not sure why it behaves differently, but it does.
Merge pull request #68 from minrk/sudo-kill-2

use `sudo kill` for signaling when subprocesses are started with sudo