Introduce send_multi and send_prepared

As discussed here:

```
https://groups.google.com/d/msg/sockjs/vsFvHqppq5g/XQPgErs9BpUJ
```

We should introduce three new calls:

```
sockjs:send_multi([Conn], iolist)
sockjs:prepare(iolist)
Conn:send_prepared(prepared)
```

Of course `send_multi` might use `prepared` underneath.
