Pull request for Lecture 2 assignments for sea-c28

The way you've defined `fibonacci()` and `lucas()` results in a (possibly) unexpected property:

    fibonacci(0) == fibonacci(1)
    >>> True
    lucas(0) == lucas(1)
    >>> True

This assignment didn't specify what values of input your functions needed to be able to handle, so there's no points lost here. I want to point it out because we're getting into assignments that deal with user input (which should never be trusted) for which you'll need to think about which input types and values should be considered valid and how to validate them.