Add Nullable version of uniqueidentifier

[jeebiq]

[denisenkom]

Why do you think this is the right way of adding new type support?

[jeebiq]

I didn't see anything in the readme about how you wished to have anything new added, so I added as new/dedicated files to keep it as simple as possible for me without touching the existing code.

[denisenkom]

You need to add a roundtrip test with the actual SQL server. One test should set a value in the SQL and read it uniqueidentifier. Another test should verify sending uniqueidentifier value into SQL server and then read it back and compare with original. Each test should test actual value and NULL.