Ed25519 Identity Keys for Mixnodes

[futurechimp]

At the moment, we have encryption keys for mixnodes, but no identity keys.

Let's implement an identity signing feature so that nodes can sign requests to the validators (and potentially other entities).

For now, just do the key generation, key persistence and signing methods. We will figure out the full signature scheme in the next few days, right now we're only setting up the infrastructure to do the signing.