Hash CRC16 support.

https://github.com/twitter/twemproxy/issues/69

implementing CRC16.

but, there are too many CRC16 implementation. and they are a little bit different.

so I just follow linux kernel's implementation. and conf is below:

```c
leaf:
  listen: 127.0.0.1:22121
  hash: crc16
  distribution: ketama
  auto_eject_hosts: true
  server_retry_timeout: 2000
  server_failure_limit: 1
  servers:
   - 127.0.0.1:11212:1
   - 127.0.0.1:11213:1
```