Query slice/index become non-generative

[jek]

__getitem__ and __getslice__ should be terminal operators on a Query

• add : into Returning Lists and Scalars of tutorial?

[Mike Bayer]

also, make sure that if the size of the slice is zero, an empty iterator is returned.

[Mike Bayer]

17684a1b81b63a436fcf4f8426bcd822bdb8d463. returns lists or scalars. Also moved the "generative" version to query.slice_(), which we can rename or hide as we deem fit. seems handy to me particularly with values().

[Mike Bayer]

Removing milestone: 0.5.0 (automated comment)