Added SQLSERVER_STRINGS macro to work around fix for #70.

* In SQL Server, SQL_WLONGVARCHAR corresponds to the deprecated ntext data type. nvarchar(max) is the preferred option.
 * To use nvarchar(max), you use SQL_WVARCHAR with a column size of SQL_SS_LENGTH_UNLIMITED (zero). This may or may not probably cause other drivers to send an empty string (?), which is why I've added this as a #define.

This isn't the best solution, since it doesn't support connecting using different drivers at the same time, but I don't know what a better solution would be (any ideas?)