Allow announce key to be missing in the torrent dictionary

[atomashpolskiy]

Per BEP-5:

A trackerless torrent dictionary does not have an "announce" key. Instead, a trackerless torrent has a "nodes" key.

But announce key is always required now.

[atomashpolskiy]

Per #103 we might need to be even more lenient and allow both announce and nodes to be missing. After all, DHT for public torrents is already bootstrapped with a few well-known nodes, and for other use cases we have programmatic API for connecting to peers dynamically, so it's probably not justified to require any static addresses in the .torrent file upfront.