Add revision log entry for mapping on FeedsNodeProcessor

The revision log entries for nodes are hardcoded on insert.

$node->log = 'Created by FeedsNodeProcessor';

and update:

    if ($this->config['update_existing'] == FEEDS_UPDATE_EXISTING) {
      $node->log = 'Updated by FeedsNodeProcessor';
    }
    else {
      $node->log = 'Replaced by FeedsNodeProcessor';
    }

Adding a target to the FeedsNodeProcessor plugin allows one to map to the node's log entry:

    $targets['log'] = array(
      'name' => t('Log'),
      'description' => t('The revision log entry for this insert or update.'),
    );