Add signature module

Because things other than transactions can be signed.

src/accountant_stub.rs

@@ -5,7 +5,8 @@
 use std::net::UdpSocket;
 use std::io;
 use bincode::{deserialize, serialize};
-use transaction::{get_pubkey, sign_transaction_data, PublicKey, Signature, Transaction};
+use transaction::{sign_transaction_data, Transaction};
+use signature::{get_pubkey, PublicKey, Signature};
 use log::{Entry, Sha256Hash};
 use ring::signature::Ed25519KeyPair;
 use accountant_skel::{Request, Response};