add method Zmadd ,with redis >= 2.4
Zrange支持 WITHSCORES 参数

demo： client.Zrange("set_key",0,-1,"WITHSCORES")
Zadd 的 批量版本，Zmadd

      val_score := make(map[string]float64)
      val_score["a"] = 1111111111111
      val_score["b"] = 2222222222222
      val_score["c"] = 33333333333333 
      val_score["d"] = 444444444444444 
  
      client.Zmadd("set_key",val_score)
Merge pull request #1 from redoufu/master

zadd multiple elements. redis >=2.4
Merge pull request #2 from liufuqiang/patch-1

add method Zmadd ,with redis >= 2.4
Merge pull request #3 from liufuqiang/master

修改redis 的 Zrange 方法，支持WITHSCORES； 增加Zmadd方法
Merge https://github.com/astaxie/goredis