Fix PHP access level errors

Change visibility of instance variables $_db, $_table and $_columnMap from private to protected to resolve the following PHP errors:

PHP Fatal error:  Access level to FireGento_Logger_Model_Db::$_db must be protected (as in class Zend_Log_Writer_Db) or weaker in /MAGENTO_ROOT/app/code/community/FireGento/Logger/Model/Db.php on line 29
PHP Fatal error:  Access level to FireGento_Logger_Model_Db::$_table must be protected (as in class Zend_Log_Writer_Db) or weaker in /MAGENTO_ROOT/app/code/community/FireGento/Logger/Model/Db.php on line 29
PHP Fatal error:  Access level to FireGento_Logger_Model_Db::$_columnMap must be protected (as in class Zend_Log_Writer_Db) or weaker in /MAGENTO_ROOT/app/code/community/FireGento/Logger/Model/Db.php on line 29