Added _masters method

Added a helper method RedisCluster::_masters to get every master
that phpredis knows about, for the purposes of things like
scanning across the whole keyspace
SCAN command

Initial commit of the SCAN command for cluster.  As a first iteration,
the command takes a host:port string (which can be retreived from the
introspection method RedisCluster::_masters()), to scan either
everything in the cluster (e.g. in a loop), or one in particular.

A simple way to scan everything in the whole cluster Keyspace

$obj_c->setOption(RedisCluster::OPT_SCAN, RedisCluster::SCAN_RETRY);
foreach($obj_c->_masters() as $str_hash) {
    $it = NULL;
    while($arr = $obj_c->scan($it, $str_hash)) {
        foreach($arr as $key) {
            echo "$str_hash -> " . $key . "\n";
        }
    }
}