Property last_update, moved connecting code to separate public method

1: Property last_update contains (after connect) date and time of last update of database. It can be useful for decide whether to start().
2: I moved the connecting code to separate public function, because last_update can only be filled after connecting.

Example:
$dump = new MySQLDump($db, $user, $pass, $host);
$dump->connect();
if($dump->last_update > $last_backup || !$last_backup) $dump->start("$filename");