Improved vdf parsing

I added support for non-quoted keys/values and ignored bracketed values. (backported from my [hasty port to coffeescript](https://github.com/forivall/vgui.js/blob/master/src/vdf.coffee)).

If you want, I can also write some tests for this (since I'll also need similar tests for my parser). I did some basic smoke tests by adding a `'__main__'` section and parsing `hudlayout.res`.