Fix ip & tcp header calculation in memcache command

Hi, it's my first time to use pull request.

I'm impressed by memkeys's work. 
Since we also need it to find hot keys, I studied it a little bit and focused on the memcache command processing. 

I found it ignores some part in IP & TCP head. Although, it's not a big deal, as it only accept alpha bytes in following 'makeResponse' function.

But I need to process binary protocol, so I fixed it. Hope it helps. 
And I also implemented mget support and binary protocol support. If you need it, I can send those as pull requests too.