NixOS integration pieces. 

Beware - there are tied to trunk as we've not yet done a release.

Use the commands:
<pre>
nix-env --install  nix-prefetch-svn
nix-prefetch-svn subversion/git-URL
</pre>

to regenerate the checksums. Once we have a release we can use proper
checksums & signatures.

To reformat/cleanse:
<pre>
for i in pkgs/servers/http/apache-modules/mod_*/default.nix; do nixpkgs-fmt $i; done
</pre>