Added PostgreSQL support for plusminus_tally

[kevinelliott]

Database agnostic type_for_sql injection for typecast in select's SUM.
Use the provided column_names_for_tally for enumerating the current table's column names since PostgreSQL requires it when you use aggregate functions (this could alternately be used in the select instead of the group).

[bouchard]

Sorry, MySQL doesn't like that, still not sure how to do this properly between Postgres / MySQL. Error is:

ActiveRecord::StatementInvalid: Mysql2::Error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'int(11)) WHEN 1 THEN 1 WHEN 0 THEN -1 ELSE 0 END) AS plusminus, COUNT(votes.id) ' at line 1: SELECT items.*, SUM(CASE CAST(votes.vote AS int(11)) WHEN 1 THEN 1 WHEN 0 THEN -1 ELSE 0 END) AS plusminus, COUNT(votes.id) AS vote_count FROM `items` LEFT OUTER JOIN votes ON items.id = votes.voteable_id GROUP BY items.id, items.user_id, items.name, items.description ORDER BY plusminus DESC