Update to Listen 2.0 (still in beta)

Here the update for the new Listen 2 API. 

As version 2 is still in beta stage (*BSD and Windows adapter hasn't been tested) I'm not sure if it's a good idea to include it directly in Guard 2.0, maybe waiting for 2.1 would be a safer idea. 
In an other side, it would be way kicker tested/used if included in Guard 2.0 release.

What do you think guys?