Create encryptor and decryptor based on XOR decryption

- Project Euler Problem 59 defines an encryption method
  where each byte of a message is XORd against a a corresponding
  value in an arbitary key.  Since XOR is a symmetrical operation
  (not sure if thats a legitimate word to describe it), you can
  XOR back from your encrypted message to the original message
  providing you know the corresponding value in the key.
  Works great if the key is the same length or longer than the
  original message.

- This will be incorporated into the Limey gem at a later date.