added gpodder package

Hi,

I added a gpodder package. Please keep in mind, this is my first package. So if something is not optimal please notify me and I happily fix it.

Best,
Sven
@lethalman The order should be irrelevant. But anyways, the best approach would be to use `makeFlags`, e.g.

    makeFlags = [ "DESTDIR=/" "PREFIX=$out" ]

and to use `preFixup` for the `wrapProgram` call. Then `installPhase` does not need to be overwritten.