[fix #13] Fix handling of reduced view results

Reduced view results do not have `total_rows` nor `offset` fields and
rows have no `id` field.

The current view API is not really satisfying.

For example the value type of a view may differ whether it is reduced or
not. Why should I decide this type before I query the view? Or if I have
to choose types this soon, why can't I specify the value type after
`map` and after `reduce` instead?

closes #13