Giving hook_nodewords a little more credit and ability

- This patch firstly gets nodewords to use it's own hook to generate the metatags. This is just a cleanup of the code and means it's a more consistent implementation across multiple modules. It also means that nodewords_nodewords() can act as an example for other modules.
- Secondly it gives the hook_nodewords() function an operation called 'list' which lists the tags which become available from that module. This allows a module to define it's own metatags (like in our case, an AGLS module can define the tags for AGLS metadata). This is integrated into the nodewords API so that these can be selected/deselected from the metatags configuration page.

Together these changes mean I can do this (and define my own tags, with correct integration):

function funnelback_nodewords(&$tags, $op, $type, $ids) {
  switch ($op) {
    case 'list':
      return array('Funnelback.og');
    case 'prepare':
      if ($type == 'node') {
        $node = node_load($ids[0]);
        $tags['Funnelback.og'] = (is_array($node->og_groups_both)) ? implode(', ', $node->og_groups_both) : 'Public';
      }   
      break;
  }
}

There are more patches I'd like to make, but getting this accepted would be a first step. Including moving the DC.title tag out into it's own local plugin (metatags/title.inc).