improve Event section in Tutorial

[LianaHus]

ethereum/wiki#23

[taoeffect]

Thanks for keeping this open. 👍

[itsnotlupus]

On that line of thought, it'd be great to have something explaining how long transaction logs are kept around to be accessed by web3's filters or similar mechanisms.

I thought they were permanent, but the 256 block limitation on accessing block hashes makes me wonder if that's really the intent.

The persistence of events can make a fairly large difference in the way external code will interface with contracts, so clarity there would be really useful.

[sillytuna]

I asked Vitalik about this. He said logs are stored forever although in future it'll be down to individual nodes (who I suggested could just for archive access). The longevity of logs and contracts is important to DAPs so I would also like to see more documentation on this.