Fix for inserting unicode string into MsSql

Fix for this: https://github.com/ServiceStack/ServiceStack.OrmLite/issues/69
Solution: simply all  strings are prefixed with "N" for MsSql.
A failing test is in ServiceStack.OrmLite.SqlServerTests / UnicodeTests.cs (already pulled in)

All sql server tests are still OK, same execution plan even if the real table column is declare as varchar, not nvarchar.