Changed to allow replication connection

This change allows replication connection.

When you want to use replication feature, you can set the flag in db.php.

```php
<?php
return array(
	'default' => array(
		'connection'  => array(
			'dsn'        => 'mysql:host=localhost;dbname=default',
			'username'   => 'dbuser',
			'password'   => 'dbpass',
		),
	),
	'db1' => array(
		'type'        => 'pdo',
		'connection'  => array(
			'dsn'        => 'mysql:host=localhost;dbname=db1',
			'username'   => 'dbuser',
			'password'   => 'dbpass',
		),
		'identifier'   => '`',
		'table_prefix' => '',
		'charset'      => 'utf8',
		'enable_cache' => true,
		'profiling'    => false,
	),
	'db2' => array(
		'type'        => 'pdo',
		'connection'  => array(
			'dsn'        => 'mysql:host=localhost;dbname=db2',
			'username'   => 'dbuser',
			'password'   => 'dbpass',
		),
		'identifier'   => '`',
		'table_prefix' => '',
		'charset'      => 'utf8',
		'enable_cache' => true,
		'profiling'    => false,
	),
	'replication' => array(
		'enable' => true,
		'master' => array('db1'),
		'slave' => array('db1', 'db2'),
	)
);
```

Only Database_Query_Builder_Select uses the slave connection.
And other queries use the master connection.

Respectively, specify the multiple connection name, the destination is a random lottery.

I want your opinion.