Implements the Casesar Cipher in the class CaesarCipher.
An object of CaesarCipher is instantiated with the following hash: :phrase => <string>, :key => <n>
where n is a number greater than 0. An empty string is not encoded
and using a key of 0 will simply return the given phrase.
One note: passing a key of 26 will give you the same phrase back due
to the wraparound.