Addes supporto to dblib driver

Created a dedicated "dblib" driver configuration class.

Indeed, when tring to connect to a MSSQL server from Linux using the "sqlsrv" driver there was a problem in SqlSrvConfig.php:

    $params['server'] = $params['host'];
    unset($params['host']);

According to [PDO specs](http://www.php.net/manual/en/pdo.construct.php), the DSN string needs the "host" param, not "server".