pythonPackages: new pypi2nix implementation

The new pypi2nix implementation (maybe the last one :P), **do not merge it yet!**

**Project page:** https://github.com/offlinehacker/pypi2nix

This will be released soon, but currently it's up to nix/python developers to **test it and report any problems**. Also buildout versions parsing support has to be added asoap( @garbas ?). There's also one new file **python-overrrides.nix** where you speciffy nix overrides for generated packages. For additional documentation please reffer to project page.

To install:
- merge/cherry-pick this branch
- install `pythonPackages.pypi2nix`

To test:
- run: `pypi2nix pkgs/top-level/python-packages.json pkgs/top-level/python-packages-generated.nix`
- update: `pypi2nix --update pkgs/top-level/python-packages.json pkgs/top-level/python-packages-generated.nix`

@garbas @iElectric @matejc you might be intereseted.

Thanks for all the support :)