Make decoder callback based

[rauchg]

It should emit a callback with each packet:

decodePayload(packetsString, function(packet, isLast){});

Callback arguments:

• packet just like each item of the returned array right now
• isLast true if it's the last packet in the payload, false otherwise.

Instead of returning an array like it is now.
This will give us the possibility of giving events to the user before all packets are parsed from the string (important for very big payloads).

Let's make sure to add tests for this.

[rauchg]

@sweetieSong is gonna tackle this.

[defunctzombie]

This is likely done when @rase- did the work for blobs.