Add the load of the races respository files.

The main idea here is add some convention to add new content data for races, items, character classes, backgrounds and etc.

I used a little bit of meta programming. XD
I think that this is out of convention. Each file doesn't generate a class.

`RaceRepository::Dwarf` doesn't exist for example.

I don't know how complex it will get, but so far it is pretty simple so a pretty straight-forward approach would be simpler.

I'd keep the same implementation as the `race.rb` by just moving the constants to this model, and keep the `all` with the same implementation.

Why splitting the races into files? What is bothering you?