Adding wp-config.php Back Into Version Control

I don't agree that we should ignore the wp-config.php file. 

* The config has important information for running the site; more than simple DB credentials
  * [Multisite](http://codex.wordpress.org/Editing_wp-config.php#Enable_Multisite_.2F_Network_Ability)
  * [Caching](http://codex.wordpress.org/Editing_wp-config.php#Cache)
  * [Language] (http://codex.wordpress.org/Editing_wp-config.php#Language_and_Language_Directory)
  * [Post revisions](http://codex.wordpress.org/Editing_wp-config.php#Post_Revisions)
  * [etc...](http://codex.wordpress.org/Editing_wp-config.php)
* Developers, including myself, use [local config files](http://markjaquith.wordpress.com/2011/06/24/wordpress-local-dev-tips/) to override specific variables in the production/testing config files where we need to

See [pull 310](https://github.com/github/gitignore/pull/310) for when the wp-config was removed.