Add ResultRow.IsNil() + ResultRows.IsNil() + minor doc updates

Fix for #11

	row := r.Table(table).Get("missing key").RunRow(rs)
	if !row.IsNil() {
		err := row.Scan(obj)
	}

	rows, err := r.Table(table).Get("missing key").Run(rs)
	for rows.Next() && !rows.IsNil() {
		err := rows.Scan(obj)
	}

I did not add rows.IsEmpty() because I found it simpler to test if the current row is nil.