implement ZISMEMBER for zset

ZISMEMBER for zset are same as SISMEMBER for set: it determine if a given value is a member of a zset.

Format:

``ZISMEMBER <key> <member>``

Example:

    redis 127.0.0.1:6379> FLUSHDB
    OK
    redis 127.0.0.1:6379> ZISMEMBER number pi
    (integer) 0
    redis 127.0.0.1:6379> ZADD number 3.14 pi
    (integer) 1
    redis 127.0.0.1:6379> ZISMEMBER number pi
    (integer) 1
    redis 127.0.0.1:6379> ZISMEMBER number e
    (integer) 0