Removed cast to Comparable when adding database objects to a Tuple.

Currently it is not possible to read rows from a PostgreSQL database
that are org.postgresql.util.PGobject instances, since they don't
implement java.lang.Comparable. Removing this type cast fixes this.
Merge pull request #39 from r0man/remove-comparable-cast

Removed cast to Comparable when adding database objects to a Tuple.