add support for empty constructor

now you can edit

...
	protected $database_type = 'mysql';
	protected $server = 'localhost';
	protected $username = 'username';
	protected $password = 'password';
	protected $database_name = 'database_name';
...

and run

$medoo1 = new medoo();
$medoo2 = new medoo('another_database');
$medoo3 = new medoo($options);