Altcoins - a new category of applications

Here, I present a new level of hierarchy on Nixpkgs: the Altcoins.

The idea is to put a single cryptocurrency per file, and build them
using expressions like altcoins.bitcoin. I believe this ordering is
clearer and more maintainable that the current one.
Well, I read the code of Gnome 3, and it uses a similar structure: inherit some variables, and use them on the derivations above. If this is not necessary, I don't know. But it doesn't hurt me anyway :)