3.0 plugin ns

Allow loading Plugins with namespaces of the form 'Author\PluginName', and make it convenient to do so via `Plugin::load('Author\PluginName')`.

Also verify that a class loader is added with the correct path - previously classes in `Author\PluginName` named plugins were not possible to load ( the registered path would be `.../Author` leading to Cake trying to load `.../Author/Author/PluginName/.../Foo.php`).