Use secure connection, add group identification

[mtwebster]

• All machines that wish to send files amongst themselves must have the same 'group code'.
• Server generates its own self-signed cert and private key. The cert is encrypted with a sha256 hash of the group code and is embedded in the zeroconf service info.
• Client discovers server, extracts cert and must be able to decrypt (with the same code) before connecting is possible.
• If codes match, the cert can be retrieved by the client and connection proceeds.