elide _id when reading from collections (Fixes #123)

Note: this means you don’t see `_id` when just reading from
user data, and you get one less column than expected if
you run a query like `select _id, city, state from zips`. We’ll
address those problems in a future fix. See #287.
Merge pull request #434 from mossprescott/ready/123

elide _id when reading from collections (Fixes #123)