Output signal strength (RSSI) for discovered devices
Currently, the plugin only logs the name and address when a new BLE device is discovered. Per discussion in #2 it would be helpful to output the RSSI (signal strength in dB) as well.

![console](https://cloud.githubusercontent.com/assets/8467740/20715453/df4e4c6e-b614-11e6-9d4a-8671dd7487a6.png)

RSSI can be used to tell which peripheral device is closer to the central and also detect possible problems with a weak signal.