Add support for Client.add callbacks.

convenience callback.
Sometimes you want a callback for a specific torrent.

Alternatively, instead of doing:
```js
Client.add(link)
Client.on('torrent', function(torrent) { ... }) // can be any torrent
```
do:
```js
Client.add(link, function(torrent) { ... }) // cb specific for this torrent
```
callback is run before Client's 'torrent' event.