Example doesn't work without message

The example:
```bash
   $ notify -t "Agent Coulson" --icon https://raw.githubusercontent.com/mikaelbr/node-notifier/master/example/coulson.jpg
``` 
doesn't work unless a message is also included:
```bash
   $ notify -t "Agent Coulson" -m "It's a magical place" --icon https://raw.githubusercontent.com/mikaelbr/node-notifier/master/example/coulson.jpg
```
