Add support for PostgreSQL in Adapter\PDO\Sql

The PostgreSQL support of Adapter\PDO\Sql is incomplete.

PostgreSQL is obviously a SQL database, but it has the different syntax from MySQL:
- Identifiers can be quoted by ```"```(doublequote), not by ``` ` ```(backquote)
- ```INSERT INTO table SET column = value``` is not suppoerted.
- ```SHOW TABLES``` is not supported.
- The view ```pg_tables``` has the info of each table.
- The column ```tablename``` in ```pg_tables``` has the name of each table.

This PR adds the PostgreSQL specific codes.