Add optional callable to drive extraction of node hash in distributor.

This is mainly in response to the longstanding issue #36 in which my
proposed solution was fine in terms of functionalities, but eventually
never made into the repository since it was far from being clean enough
for my taste.

Now developers can optionally pass a callable object when creating the
hashring instance to decide how the distributor should extract the hash
from a node (really a connection instance) to populate the ring:

  use Predis\Cluster\Distribution\HashRing;
  use Predis\Connection\PredisCluster;

  $servers = array(
    'tcp://10.0.0.1?alias=node01',
    'tcp://10.0.0.2?alias=node02',
  );

  $options = array(
    'nodehash' => function ($connection) {
      return $connection->getParameters()->alias;
    },
    'cluster' => function ($options) {
      $replicas = HashRing::DEFAULT_REPLICAS;
      $hashring = new HashRing($replicas, $options->nodehash);
      $cluster  = new PredisCluster($hashring);

      return $cluster;
    },
  );

  $client = new Predis\Client($servers, $options);

Both HashRing and KetamaPureRing in the Predis\Cluster\Distribution
namespace support this new approach.