Merge pull request #6 from andybalholm/master

Handle more field types.
Merged #7 . thanks @doun
Fixed #8
Move CoInitialize into driver.Open. related issue #12
Add CoUninitialize at conn.Close(). related issue #12
Merged @andybalholm's patch: Related issue #13
Check for NULL values before believing the column type.

No matter what type the column says it is, the value could be a NULL,
so we need to check the variant type first. Otherwise we read nonsense
data.