As a programmer
So that the game can be played according to the rules
I want a normal number to return that number

As a programmer
So that the game can be played according to the rules
I want a number divisible by three to return 'fizz'

As a programmer
So that the game can be played according to the rules
I want a number divisible by five to return 'buzz'

As a programmer
So that the game can be played according to the rules
I want a number divisible by fifteen to return 'fizz buzz'

As a programmer
In order to work in a test driven way
I would like to run unit tests

Extra credit:

As a programmer
In order to handle "irregular" user behavior
I want a non-numetical input to return 'the input must be a positive number!'