Add gzip support for channel unpacking 

I need this, because in my pet project ceh (http://github.com/errge/ceh) I download nixpkgs snapshots from github instead of http://nixos.org/releases/nixpkgs/...

At nixos.org, old snapshots are continuously garbage collected, while github has the whole history of the project and I'd like to have old versions installable too.

Unfortunately github only offers gzip and zip compression for archive downloads.  I think this is because, both bzip2 and xz is a bit expensive to compress.

Currently I have a big hack to first download the gzip, then recompress to bzip2 on the client side.  That of course is a big performance hit for my users.