Enable connection class retargeting

[benlower]

Take an existing TDS file ( say for PostgreSQL DB ) and change connection to MySQL/SQLServer/etc DB connection by changing the connection attributes from (class='postgres' to class='mysql' ) , Servername, dbname, port, username, auth-mode, sslmode, schema, single-node, workgroup-auth-mode.

[t8y8]

If we decide to allow this, it's just a matter of making the class attribute settable (Currently it's read only)

[t8y8]

I'm going to consider this closed by #65 and we can do new issue reports for the odd-ball attributes that specific connectors have