[RFC] Doxygen neovim.org theme

A picture is worth a thousand words:
[![devdocneovimtheme](https://cloud.githubusercontent.com/assets/393493/3210941/9ec49c30-eef1-11e3-8a61-96954a802e12.png)](http://neovim.menkar.uberspace.de/devdoc/neovim_theme/fs_8c.html)

http://neovim.menkar.uberspace.de/devdoc/neovim_theme/

ref https://github.com/neovim/neovim.github.io/issues/48
All of these newly-added files should probably go in `contrib/`: https://github.com/neovim/neovim/tree/master/contrib

There was a long discussion about where to support files for third party tools, and that's what we landed on. It would be confusing to have a `documentation/` directory which does not actually contain documentation in it (probably only the build-bot will, in the future :)