Fix for issue #428: Use PLUGIN_PATH properly

Fixes issue #428 by temporarily adding ``PLUGIN_PATH`` to the ``sys.path``.

Another change: ``.plugins`` attribute of the ``pelican`` object was just a duplicate of ``PLUGINS`` option (i.e. strings, module objects, whatever you put there). Now it's a list of loaded module objects. I believe this is more consistent. 

It does not break anything in the Pelican core, since nothing internal cares about that attribute. If there is a custom plugin out there relying on that attribute, it might be 'breaking;. But I have no idea what anyone would do with that attribute in its old state.