Support domain mapping

I installed [WordPress MU Domain Mapping](http://wordpress.org/plugins/wordpress-mu-domain-mapping/), but git sees `www/wp-content/sunrise.php` as an untracked (but trackable) file. Adding the `SUNRISE` constant is easy, since it can go in `www/local-config.php`.

One possible solution that I came up with is to add a line to `.gitignore` to ignore `www/wp-content/sunrise.php`. I wanted to open an issue first, though, so that solution or any alternatives could be discussed before a patch is written.