Add methods for float multiplication/division

[JamesGuthrie]

It would be very convenient to have the methods mul_f32, mul_f64, div_f32 and div_f64, which are available on std::time::Duration available on chrono::Duration.

To be clear it is possible to round-trip through std::time::Duration which I'm currently doing:

let duration = chrono::Duration::from_secs(2);
let multiplier = 0.5;
let half_duration: chrono::Duration = chrono::Duration::from_std(duration.to_std().unwrap().mul_f64(multiplier)).unwrap();

But it would be nicer to do this:

let duration = chrono::Duration::from_secs(2);
let multiplier = 0.5;
let half_duration = duration.mul_f64(multiplier);

[djc]

Sounds reasonable, if you submit a PR against the 0.4.x branch I'll review it.