Change action autoloading to allow actions in plugins

Change action autoloading to allow actions in plugins

Since plugins may define custom actions, we shouldn't require that
there be a file in our actions/ subdir for every action. So, I changed
the (admittedly hackish) auto-loading code in index.php so it instead
checks whether a class exists with the expected name. This, in turn,
uses the increasingly hacking __autoload() function, which I changed
to auto-load stuff named "BlahblahAction" from the actions subdir if
available.