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>
Simple SELECT aggregate implementation

This modified generators/sql-val to treat '(sum :price) specially: the
first member will be the aggregate function, the rest, the parameters.

SPEC updated, and a test case added.

This fixes #1.

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