Add a way to pass DSN to pyodbc in mssql

[Former user]

It would be very useful to have a way to pass DSN to pyodbc driver instead of URL with host/uid/database

[paj]

Yes, how about the URL syntax mssql:///?dsn=fred

I'll code it so if you specify a DSN, the server/port/database options are ignored. I guess it should still pass the username and password.

[paj]

Ok, this is done, changesets 3347 and 3348