Add method `is_one` to `One` trait

The [`Zero` trait](http://rust-num.github.io/num/num/trait.Zero.html) contains two methods: `zero` returns the number 0 for the given type and `is_zero` is provided and can be used to check if a number is 0.

The [`One` trait](http://rust-num.github.io/num/num/trait.One.html) is similar but contains only the method `one` to create the number 1. Please also add a method to check if a number is one.
