minor fixes:
- restored getEmailClient and setEmailClient in Namshi\Notificator\Notification\Handler\EmailVision.php
- Namshi\Notificator\Notification\Handler\NotifySend::handle returns the output
- removed blank line in Namshi\Notificator\Notification\Handler\NotifySend
- add phpdoc to Namshi\Notificator\Exception\ExecutableNotFoundException