Added mysqli driver.

Portability column forced to lowercase

When using a PDOConnection and the Portability wrapper, the default value \PDO::CASE_NATURAL int(0) and PHP constant CASE_LOWER int(0) got mixed up.

Fixed a few cases of where MysqliStatement::execute() did not throw a exception if a error had a occured.

Hopefully final fixes. Added phpdoc @inheritdoc where it were missing. Added MysqlConnection::getWrappedResourceHandle(). MysqliStatement::fetchAll() now decide fetch style if none is set, instead of letting fetch() do it on every call.

This enables a more natural way of interacting with result sets,
and re-introduces in Doctrine DBAL a feature that has been available
in PDO from the begining.