Update Nginx conf template to match Boxen default better

Felt the Nginx config do with a bit of an update and at the same time keep it a bit more consistent with the default Boxen one.

The bad news (breaking changes):
 - Expects a `public` directory not `www` – change because it is consistent with default Boxen template

The good new (yay cool stuff):
- Using instance variables to get rid of deprecation notices
- Sets index files
- Wildcard for all subdomains under `<project name>.dev`
- Works with [xip.io](http://xip.io/)
- Takes care of `50x` error pages in the same way as default Boxen
- Specific block for catching `.php` scripts with an upstream location