Add math::abs function

[ISibboI]

This is a great idea! Could we add it in the way that it returns integers if the input value is integer? Since abs can be defined both on integers and floats.

[LinuxHeki]

Sure we can. This would probably require more than just one line of code.

[LinuxHeki]

Done! It was easier than I thought

[ISibboI]

Great thanks, I can take it from here.

[ISibboI]

If you want me to release this now, please just make a comment. Otherwise, I'd release it together with #131.