Add @return type hint

Close #714

#714 - ignoring PDOExceptions on PDO#getAttribute(PDO::PGSQL_ATTR_DISABLE_PREPARES) as per https://bugs.php.net/bug.php?id=68351

#714 - Adding a test verifying that attribute `PDO::PGSQL_ATTR_DISABLE_PREPARES` is set when using pgsql on PHP 5.6+

Fix tear down foreign key constraint violation exception tests

[DBAL-1024] Add more foreign key constraint violation error codes

[DBAL-1028] Fix fetching NULL values via fetchColumn()

[DBAL-1029] Fix foreign key referential action SQL on SQL Server and SQL Anywhere

[DBAL-1023] Fix line separators

[DBAL-1025] Allow connecting without database name for sqlanywhere driver

[DBAL 930] Only introspect accessible schema objects in PostgreSQL

[DBAL-944] Fix table column alteration on DB2

Fix return type of Connection::project

Add test to verify null cast in boolean type

Update from master main repository

fix quoted sequence name

[DBAL-1022] Wrap PDOException in PDOConnection::exec()