Logger

Added support for multiple loggers in application. 
Configuration should be like this:

```php
'logger' => array(
    'Backend\Logger' => array(
        'writers' => array(
            array(
                'name' => 'stream',
                'priority' => 1,
                'options' => array(
                    'stream' => __DIR__ . '/data/log/backend.log',
                ),
                'filters' => array(
                    array(
                        'name' => 'priority',
                        'options' => array(
                            'priority' => E_STRICT | E_WARNING | E_NOTICE
                        ),
                    ),
                ),
                'formatter' => array(
                    'name' => 'base',
                    'options' => 'Y-m-d H:i:s',
                ),
            ),
        ),
    ),
    'Frontend\Logger' => array(
        'writers' => array(
            array(
                'name' => 'stream',
                'priority' => 1,
                'options' => array(
                    'stream' => __DIR__ . '/data/log/frontend.log',
                ),
            ),
        ),
    ),
)
```