Checks if a nick is on or not.
        Returns list of nicks online.
        Required arguments:
        * nick - Nick to check.

def ison(self, nicks):
        """
        Checks if a nick is on or not.
        Returns list of nicks online.
        Required arguments:
        * nick - Nick to check.
        """
        with self.lock:
            self.send('ISON :%s' % ' '.join(nicks))
            online_nicks = []
            if self.readable():
                msg = self._recv(expected_replies=('303',))
                if msg[0] == '303':
                    online_nicks = msg[2].replace(':', '', 1).split()
            return online_nicks