Provide functions to get public keys from secret keys.

[dchest]

nacl.box.keyPair.fromSecretKey(secretKey) => // returns key pair
nacl.sign.keyPair.fromSecretKey(secretKey) => // -"-

Implementation of the first one is easy:

exports.box.keyPair.fromSecretKey = function(secretKey) {
  checkArrayTypes(secretKey);
  if (secretKey.length !== crypto_box_SECRETKEYBYTES)
    throw new Error('bad secret key size');
  var pk = new Uint8Array(crypto_box_PUBLICKEYBYTES);
  crypto_scalarmult_base(pk, secretKey);
  return {publicKey: pk, secretKey: secretKey};
};

~~Sign will be ~copy of crypto_sign_keypair, which is ugly, but I don't want to refactor tweenacl.c-based code.~~ (Don't file issues when ill! Sign public key is just the second 32-byte part of the secret key).