A first go at persisting validator keys and handling configuration. Addresses issue #253.

- Creates a keystore directory in the config
 - Fetches serialized keys from the keystore directory
 - If no keys, generates keys randomly, saves serialized keys to keystore dir.

eth2/utils/bls/src/keypair.rs

@@ -14,4 +14,8 @@ impl Keypair {
         let pk = PublicKey::from_secret_key(&sk);
         Keypair { sk, pk }
     }
+
+    pub fn identifier(&self) -> String {
+        self.pk.concatenated_hex_id()
+    }
 }