Zend\Log\Formatter\Xml improvement to handle extra data array

I found out, that the \Zend\Log\Formatter\Xml doesn´t support  "extra" information, when its a extra data is an array. I made an example to show my little change.

Hope its useful.

Example-Code:
```php
$stream = new \Zend\Log\Writer\Stream('data/log/application.log');
$stream->setFormatter(new \Zend\Log\Formatter\Xml());
$logger = new \Zend\Log\Logger();
$logger->addProcessor(new \Zend\Log\Processor\Backtrace());
$logger->addWriter($stream);

$logger->info('wrong stuff');
````

before:

```xml
<logEntry>
   <timestamp>2014-04-14T23:40:17+02:00</timestamp>
   <priority>6</priority>
   <priorityName>INFO</priorityName>
   <message>wrong stuff</message>
</logEntry>
```

after:
```xml
<logEntry>
  <timestamp>2014-04-14T23:40:17+02:00</timestamp>
  <priority>6</priority>
  <priorityName>INFO</priorityName>
  <message>wrong stuff</message>
  <extra>
      <file>/home/project/module/App/src/App/Controller/ConsoleController.php</file>        <line>51</line>
      <class>App\Controller\ConsoleController</class>
      <function>loggerTestAction</function>
  </extra>
</logEntry>
```