Add dblib/freetds connect as SQL Server option

PDO_SqlSrv is only available on Windows machines.  This ads the ability connect through dblib/freetds on Unix platforms as a failover to the Windows method.  

Note: This this pull request duplicates some of the connect() code so this can probably be abstracted/improved further.  For now, it allows Unix machines to connect to SQL Server, which is important in IT environments using multiple platforms.