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.
update js_of_ocaml to 2.5
update tyxml to version 3.3.0
add version number to the name of the js_of_ocaml package
Updgrade ocsigen-server to 2.5
Update eliom to 4.1.0
qtcreator: Update from 3.2.0 to 3.2.1
Make uutf a *propagated*BuildInput of ocamlPackages.tyxml
Build js_of_ocaml with tyxml support
Adapt the derivation for the update of eliom to 4.1
Update firefox-bin to 33.0
eggDerivation: Move postInstall into installPhase.
firefox-bin: uses sha1 provided by Mozilla
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.