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>
Merge pull request #12 from algernon/f/select-aggregate

Implement SELECT aggregates