Change game speed with numpad

I noticed that I wasn't able to change game speed via the key shortcut (although I could pause the game by pressing P) and it was a bit frustrating. I had a look into the code and I realized that shortcuts handling assumes a qwerty keyboard. It doesn't work for me because I'm using an azerty keyboard.

That seemed a bit complicated to change, so I thought it would already be a bit better to be able to use numpad keys to change game speed.

This is the very first time I'm using git and doing anything in Lua, so it is indeed a modest contribution. I hope I'll have some time on the occasion to further contribute.
How about a function that checks if it is a code from numpad returning a boolean. And then another function which processes that value, then you don't have to return -1, nil or false. Because your function does 2 things it suggest they can be decomposed. 

```lua
if code_from_numpad(code) then
      process_numpad_code(code)
end 
```
Could you please also document your functions the agreed style is found at the bottom of this page:
https://github.com/CorsixTH/CorsixTH/wiki/Coding-Conventions