Change sum-of-multiples exercise to require a function instead of a class

[sjakobi]

At the core of the exercise is a simple mapping from a collection of numbers to a sum of multiples of those factors. This can be represented as a simple mathematical function and doesn't require any manipulation of an object.

The test suite should therefore require (and import) a function sum_of_multiples instead of a class.

For some discussion regarding the exercise see #139.

[kytrinyx]

+1