oj/core.clj: Trim extra spaces in sqlify

When SQLifying an expression, if any of the generators returned nil,
skip that entirely, instead of adding a space.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Add a group-by generator

This adds a group-by generator, along with documentation and a test
case.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>