Added database adapter abstract service factory.

Added database adapter abstract service factory to configure multiple database adapters for application. This abstract factory reserved *db.adapters* key in configuration array.
```php
array(
            'db' => array(
                'adapters' => array(
                    'Zend\Db\Adapter\Master' => array(
                        'driver' => 'mysqli',
                    ),
                    'Zend\Db\Adapter\Slave' => array(
                        'driver' => 'mysqli',
                    ),
                ),
            ),
)
```