Feature/log writer mail - 

Hi,

This patch allow the creation of an Zend\Mail\Message in Zend\Log\Writer\Mail through options passed to the writer. if $mail['mail'] is an array, we create a Zend\Mail\Message.

Here is a example of configuration, allowed by this patch
```
    'log' => array(
        'Log\App' => array(
            'writers' => array(
                'mail_log' => array(
                    'name'      => 'mail',
                    'options'   => array(
                        'mail' => array(
                            'from'      => 'system@example.com',
                            'to'        => 'admin@example.com',
                            'Encoding'  => 'UTF-8',
                        ),
                        'subject_prepend_text' => '[alert]',
                    ),
                ),
            ),
        ),
    ),
```

As this is my first pull-request in zf2, don't hesitate to give me feedback so that I can be better.

Cheers,
Patrick