Another new release!

It turns out the way I got the state of the blockchain in the past to verify fork blocks was a huge bottleneck, so I fixed it by adding some caching logic. I've also added some more queuing and deferring to prevent the client from locking up while verifying a batch of downloaded blocks.

Hence 0.4.

There's actually a reasonable 3,843-block blockchain going, on which I will pay 50 coins to anyone who posts their address and is running a client to receive them. Eventually.