Make sure that helper functions are all static. Add new

File_Structure_Test to make sure we don't regress.

According to the PHP docs, the "public" keyword is implied on static
functions, so remove it.  Also, require private static functions to
start with an _.

http://php.net/manual/en/language.oop5.visibility.php

modules/slideshow/helpers/slideshow_installer.php

@@ -18,14 +18,14 @@
  * Foundation, Inc., 51 Franklin Street - Fifth Floor, Boston, MA  02110-1301, USA.
  */
 class slideshow_installer {
-  public static function install() {
+  static function install() {
     $version = module::get_version("slideshow");
     if ($version == 0) {
       module::set_version("slideshow", 1);
     }
   }
 
-  public static function uninstall() {
+  static function uninstall() {
     module::delete("slideshow");
   }
 }