Add method to discover local bluetooth address

[ollym]

@atilag having a lot of success with your BluetoothSerialPortServer, but for my use case I also needed to know the local bluetooth address of the server.

It would be great to get this through a method. In the mean time I'm doing something like:

execSync('hcitool dev').toString().match(/hci0\s+(.+)/)[1].toLowerCase();

[ollym]

@atilag I'll take a look at doing this myself when I get a chance. Putting this here for reference:

https://github.com/pauloborges/bluez/blob/master/tools/hcitool.c#L77-L88