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, so far so good.

About the failure on Travis, I read the Travis log, and the build fails because gcc breaks.