Make the postgresql loader load $$ syntax correctly

I found it to be impossible to import postgresql triggers made with the $$ syntax, so I fixed it.
To fix it I had to copy paste the entire loader code from the abstract Db driver and added some extra parsing logic. Perhaps there is a better way to fix it, and I'm open for input on that.

This is the SQL code that didn't work:
```sql
-- test for triggers with $$ syntax
CREATE OR REPLACE FUNCTION upd_timestamp() RETURNS TRIGGER
LANGUAGE plpgsql
AS
$$
BEGIN
    NEW.created_at = CURRENT_TIMESTAMP;
    RETURN NEW;
END;
$$;
```

Without the fix I got this error in the testsuite
```
There was 1 error:

---------
1) postgresTest::testLoadDump
PDOException: SQLSTATE[42601]: Syntax error: 7 ERROR:  unterminated dollar-quoted string at or near "$$
BEGIN
    NEW.created_at = CURRENT_TIMESTAMP"
LINE 5: $$
        ^
```

With the fix the SQL code gets imported without any problems.

Please let me know what you think!