Add support for psk2 handshakes

[nsmith5]

In psk2 handshakes, the responder doesn't know which preshared key to use until after they read the first message. To support this, allow setting preshared key for responder after the first ReadMessage with a new SetPresharedKey method.

Example usage:

// Initiator knows and sets psk initially
initiator, err := noise.NewHandshakeState(noise.Config{
    CipherSuite:           foo,
    Pattern:               HandshakeIX,
    PresharedKeyPlacement: 2,
    PresharedKey:          psk,
    Initiator:             true,
    StaticKeypair:         bar,
})

// Responder doesn't know which psk to set until it knows which peer its talking to
responder, err := noise.NewHandshakeState(noise.Config{
    CipherSuite:           foo,
    Pattern:               HandshakeIX,
    PresharedKeyPlacement: 2,
    StaticKeypair:         baz,
})

// -> e, s
msg, _, _, err = initiator.WriteMessage(nil, nil)
res, _, _, err = responder.ReadMessage(nil, msg)

// Go look up which psk to use based on the peer static public key
psk  := LookUpPSK(responder.PeerStatic())
err = responder.SetPresharedKey(psk)

// Finish up the handshake
// <- e, dhee, dhse, s, dhes, psk
msg, respEncrypt, respDecrypt, _ := responder.WriteMessage(nil, nil)
res, initEncrypt, initDecypt, _ := initiator.ReadMessage(nil, msg)

// transport I -> R
msg, err = initEncrypt.Encrypt(nil, nil, []byte("foo"))
res, err = respDecrypt.Decrypt(nil, nil, msg)

// transport R -> I
msg, err = respEncrypt.Encrypt(nil, nil, []byte("bar"))
res, err = respDecrypt.Decrypt(nil, nil, msg)

Resolves #45