Fix issue #2 : support notifs (see example/simpleNotif.js)

Hello,

This PR fixes issue #2 and implements Collectd Binary Protocol for notifications.

Documentation is in the code.
An example (file example/simpleNotif.js) helps if you prefer to learn by example.

Note : I implemented severity constants like this :
```
Collectd.prototype.NOTIF_FAILURE=1;
Collectd.prototype.NOTIF_WARNING=2;
Collectd.prototype.NOTIF_OKAY=4;
```

If there is a better way to implement it, please tell me.

Regards
Yves