Implementation of auto-loading (in the leanest possible way) for a number of classes
`if (` vs `if(`
Wrap all classes in class_exists() containers
Finished implementing of autoloading of class files

Also:
* Moved some things around in the wpseo.php file as if PHP < 5.2, nothing of this plugin should be available.
=> needs some more work, we shouldn't use die here, but elegantly deactivate and show an admin notice.
* Refactored loading of admin page files from > 10 methods to one.
* Moved all deprecated functions and methods to the bottom of their files
* Fix for potential bugs if $post would not be known in the metabox class
* Bugfix - admin.php file was loaded for if `get_plugin_data()` function doesn't exist, while only plugin.php is needed