Add encryption option

[aburdulescu]

If the tracker and seeders are public(open to the internet), it would be good to have encryption for the critical messages exchanged between clients and peers and clients and tracker(ex. Chunkinfo message because of checksum).

Or maybe all messages to be encrypted.

For this each node(client, seeder or tracker) needs a public/private key pair(this would be useful for #6 also).