logstash service: make log verbosity configurable.
chicken: Implement support for bootstrap-builds

This is necessary in order to apply custom patches, as chicken needs a
bootstrap build to regenerate *.c files. This is also necessary when
building from git.
Infrastructure to build chicken eggs.
Implement patches.nix to override parts of a derivation
rename patches -> overrides
Add chicken to PATH of wrapped chicken binaries.
New package: egg2nix.

Generates nix-expressions from Chicken Scheme Eggs.
nixos-iso: Add zfs / btrfs to supportedFilesystems
eggDerivation: Move postInstall into installPhase.
Pass chickenEggs as a parameter to egg2nix.

Those eggs are specific to egg2nix - it's unwise to make them public.
Wrap chicken so it can find gcc.
egg2nix: Update to 0.2.

Also regenerate chicken-eggs.nix - the changeset is quite big, but
subsequent changes will be smaller as egg2nix now sorts its output.
smpeg: fix smpeg-config
SDL_mixer: add mp3 support
Atom: updated to 0.139.0
Fix virtualenvwrapper's PYTHONPATH
Update Hawkthorne game to v0.12.0
Added an up-to-date http debian mirror for those without ftp access.
Added: poppler-qt4 to python-packages.nix
New package: frescobaldi