Add a default “is true” selector.

Fixes #320.

This causes us to prefer `$project; $match` over a `$where` selector,
preventing us from falling into a map/reduce (as long as the condition
can be represented as a Mongo `$project` expression).

Also lets us filter on a constant … which is _maybe_ useful somehow?
Add an integration test for field comparisons.
Merge pull request #462 from sellout/ready/320-comparisons-between-fields

Add a default “is true” selector.