HardwareSystemEvent to indicate which Trezor the event from fired by

[ghost]

It would be useful if the HardwareSystemEvent gave an indication of which Trezor fired the event, specifically:
o The label of the Trezor

[gary-rowe]

I've implemented a two-tier API. The low level API covers messages from the Trezor while the high level API provides a use case oriented service and event pattern. This gives clear direction to application developers using the library what they need to do in response to the device's state.