Change ps command to lsof for kick_user

The `ps` command used to find the sshd process to kick a user contains switches that don't work across all versions of `ps`. After looking at numerous options I settled on using `lsof` to find the pids. As far as I know `lsof` is available on all unix based platforms and provides a cleaner way to filter on user and command.

This will also fix issue #11.