Feature: zaddnx - Issue #1128

New Feature : ZADDNX

1] spec
 * if data exists in ZSET, just skip.
 * if data doesn't exists in ZSET, add data into ZSET

2] Usage
```c
redis 127.0.0.1:6379> zaddnx mykey 1 one 2 two 3 three
(integer) 3
redis 127.0.0.1:6379> zaddnx mykey 3 one
(integer) 0
redis 127.0.0.1:6379> zaddnx mykey 2 one 4 four
(integer) 1
```