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
I talked with the people on #nixos about this. The makefile of gpodder contains `DESTDIR ?= /` and `PREFIX ?= /usr`. By leaving this the default, the build obviously fails. I tried to set `DESTDIR=$out PREFIX=$out`, but then you get `/nix/store/somesha-gpodder/nix/store/somesha-gpodder`. The only way I got it to work was to set it exactly to these directories.