Iterator support

[jethrogb]

I'd like to issue select queries that return all data lazily in the form of an iterator. I want to issue queries for a million rows or so and I don't want to allocate all the memory up front.

This seems to have been the way things worked, but it was changed in #132/#142. I don't fully understand the rationale for the change. I also don't care about associations for this particular query.

[vandenoever]

I'd very much appreciate the convenience of ORM without the memory allocation overhead. An iterator that returns shallow items that use &str instead of String would be good in low-memory use-cases.