Update DBManager.php - possible division by zero

if $before_value = 10 and $after_value = -10 then abs(2*((trim($before_value)+0)-(trim($after_value)+0))/((trim($before_value)+0)+(trim($after_value)+0))) cause a divide by zero.

For compare float values should use the epsilon technique - http://www.php.net/manual/en/language.types.float.php