Bootstrap: Autoload classes using a Composer-generated PHP 5.2-compatible Autoloader.

* `wp-admin` and `wp-includes` are scanned for classes to autoload
* Several 3rd-party and Ryan McCue-shaped libraries are excluded when the classmap is generated, see `composer.json`: `autoload.exclude-from-classmap`
* `wp-vendor/autoload_52.php` is included at the top of `wp-settings.php` - no changes need to be made to unit tests to include the autoloader
* An avalanche of `require()` and `require_once()` calls that loaded class files have been removed from the codebase.

The following files have been added to `svn:ignore` - they are not 5.2-compatible and fail during pre-commit:
* src/wp-vendor/autoload.php
* src/wp-vendor/composer/autoload_real.php
* src/wp-vendor/composer/autoload_static.php
* src/wp-vendor/composer/ClassLoader.php

We favor these files instead:
* src/wp-vendor/autoload_52.php
* src/wp-vendor/composer/autoload_real_52.php
* src/wp-vendor/composer/ClassLoader52.php

When new PHP classes are added to the codebase, simply run `composer install` or `composer update` from the project root to update the autoloader.

The future is now.

See #36335.

Built from https://develop.svn.wordpress.org/trunk@38399


git-svn-id: http://core.svn.wordpress.org/trunk@38340 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-mail.php

@@ -21,9 +21,6 @@ if ( ! apply_filters( 'enable_post_by_email_configuration', true ) )
  */
 do_action( 'wp-mail.php' );
 
-/** Get the POP3 class with which to access the mailbox. */
-require_once( ABSPATH . WPINC . '/class-pop3.php' );
-
 /** Only check at this interval for new messages. */
 if ( !defined('WP_MAIL_INTERVAL') )
 	define('WP_MAIL_INTERVAL', 300); // 5 minutes