Updated Open Function

I'm updating the open function because I'm using two diferent servers, one for development (windows) and the other for production (linux).
The extension sqlsrv doesn't work on linux, so I use dblib to work with MsSQL on Linux.
I also added an condition to detect if the extension is loaded or not, because the connection string is different.
Hope this helps to someone.
Merge pull request #106 from saruman/patch-3

Updated Open Function