Support for INSERT ... RETURNING queries


    Postgresql allows INSERT queries to return data via RETURNING. This is
    especially useful to return values generated by the DB such as sequence
    values and time stamps.

    This patch introduces a new return type as:

        {ok, Count, Rows}

    Providing the return value in this form most closely matches what comes
    back from pg. For the happy path case, there's little value in providing
    the count as that should match the number of rows returned; an alternate
    API would detect this case internally and return it as simply `{ok,
    Rows}`. For now, aligning with the data returned by the DB seemed like
    the right starting place.


ping @opscode/delivery @sdelano