Fix: RedisCollector does not support @ in password

In multi-instance mode, RedisCollector does not support the @ symbol appearing in a password:

    2014-09-05 04:49:59,980] [MainThread] Failed to initialize Collector: RedisCollector. Traceback (most recent call last):
      File "/usr/lib/pymodules/python2.7/diamond/server.py", line 244, in init_collector
        collector = cls(self.config, self.handlers)
      File "/usr/share/diamond/collectors/redisstat/redisstat.py", line 106, in __init__
        (nickname, hostport) = instance.split('@', 2)
    ValueError: too many values to unpack