Add "closer-but-not-exact" responses to our DHT knowledge

[tristanls]

As @skeggse suggested in #15, it may be useful/correct when dealing with responses that are not the exact match of the node we are looking for (those that point to closer nodes to ask next) to add these intermediate nodes to our knowledge of the DHT.

[tristanls]

More precisely, instead of only checking "closer-but-not-exact" responses against locally registered contacts like we do here index.js#L128:

if (Array.isArray(response)) {
    // arbiter "closer" responses against locally registered contacts
    response.forEach(function (res) {
        if (self.kBuckets[res.id]) {
            self.kBuckets[res.id].contact =
                self.arbiter(self.kBuckets[res.id].contact, res);
        }
    });

also add them to our local DHT knowledge:

if (Array.isArray(response)) {
    // arbiter "closer" responses against locally registered contacts
    response.forEach(function (res) {
        self.add(res); // add() handlers arbitration on its own
    });