Introduce basic OSX support

> For the ones with yellow iPhones.
> The ones that can't configure their Linux.
> For those who pay more money for the same shit.

My epic marketing skillz aside, this commits adds some basic support for OSX.

The installation comes with the price of a bit messier Makefile:

1. There is no 64 bit downloadable node-webkit, so you can't depend only on `uname` output to download it.
2. Even more, `uname -s` reports `Darwin` so you can't use it in the OS part of the url filename.
3. On top of that, the OSX binaries come in zip files, instead of tarballs.

To execute the app, I'm using [this] trick. That way you can just open `node-webkit.app` and it will automatically load the app.

  [this]: https://github.com/rogerwang/node-webkit/wiki/How-to-package-and-distribute-your-apps#mac-os-x