feat: add battery_percentage to bluetooth devices

[ojaskavathe]

Fixes #138

Using the Bluez Battery API to add battery_percentage to bluetooth devices.

Can be used through:

let mydevice = devices.get()[0]; // or map etc.
let battery = bind(mydevice, "battery_percentage");

[ojaskavathe]

Is anything else needed on this? @Aylur

[ojaskavathe]

@Aylur still waiting on a response.