Columns style sheet not loaded correctly

Should follow the model similar to other extensions. The current method can result in an invalid URL generated which contains the system path to the file.

This code works properly:

public function admin_enqueue_scripts()
	{
		list( , $url ) = RWMB_Loader::get_path( dirname( __FILE__ ) );
		wp_enqueue_style( 'rwmb-columns', $url . 'columns.css' );
	}