Arithmetic/rational task implementation

Implementation of http://rosettacode.org/wiki/Arithmetic/Rational. It looked like a great task for Rust, as the numeric traits in std::num make this very nice and simple.

It contains the solution to other rosettacode tasks that would be good to refactor in their own modules, like [greatest common divisor](http://rosettacode.org/wiki/Greatest_common_divisor) and [perfect numbers](http://rosettacode.org/wiki/Perfect_numbers).

I would rather use `std::{num, fmt};` but it is a matter of taste.