Add format_hexf functions

[youknowone]

I don't have a good idea for inf and nan. They generate errors for now.

using wrapper:

use hexf;

let s = format!("{:x}", hexf::Format(1.0f64));

using format function:

use hexf

let s = hexf::format(1.0f32)

[sanxiyn]

This does not build because there is no FormatHexfError.

[youknowone]

Fixed it, but still failing for Rust 1.15

[youknowone]

Will anyone confirm this is a mergable approach or not?