Implement Rollback.
Test transactions.
Test inserting a batch of rows and then committing or rolling back.
Change Fetch signature and avoid reallocating row slice each time.
Reorganize support routines out of cursor.go.
Reorganize support routines out of database.go.
Refactor Cursor.Fetch() into Cursor.Next(), Cursor.Row() and Cursor.Err().
Implement Cursor.Scan, complete with nullable types.