Listen for signal Device.OnActivePortUpdated

[kovetskiy]

Hi.

In following patch I've added listening for signal "OnActivePortUpdated" it allows to detect when headphones injected.

Thanks for the great lib.

[sqp]

Thanks for the contribution.
I tried to keep the lib simple and close to the DBus API rather than trying to abstract everything.
I hope you wasn't too surprised with the split registration interface and it's was easy to understand and implement.
Let me know if you want to add a link to your code in the usage/examples section.