sum-of-multiples: Implemented issue #141

Implemented issue #141 which requested that sum_of_multiples use a function rather than a class.

I implemented the multiples as a list because it made it easier to differentiate them from the limit when looking at the test file. In the example I set the default multiples to None to avoid a python gotcha with mutable default arguments.

http://docs.python-guide.org/en/latest/writing/gotchas/