emit event on receipt of all packets in a "message"

When A sends a msg M to B, B should be able to know when it's received the entire message M (as opposed to 'data', which is a fragment of M).

From a usage perspective, maybe something like:

``` js
a.send(aLongMessage)
b.on('message', function (msg) {
//  msg === aLongMessage
})
```
