Allow srem with multiple params

Allow srem with multiple params to work.

Follows the same idea of redis-rb#srem:
https://github.com/redis/redis-rb/blob/master/lib/redis.rb#L1239-L1247

Example:
```
$redis.sadd('key1', 'a')
$redis.sadd('key1', 'b')
$redis.sadd('key1', 'c')

$redis.srem('key1', 'a') 
=> true

$redis.srem('key1', 'b', 'c')
=> 2
```

Make this really fast because I needed... if you find something that could be done better, or any idea.. let me know.

Thanks
Did a quick benchmark on ruby 2.1.2 and it seems there's not much between them at all (although `===` is very marginally faster in my bench). I'll merge this as-is, it's performant :wink: