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.