External volumes are not always mounted at boot

[galaxy4public]

@netj reported that for an external encrypted volume BootUnlock is doing a poor job of reliably mounting such a volume at boot. The root cause for such a behaviour is that BootUnlock hooks in at the boot up sequence and fires just once. However, some external volumes may take time to initialise and be available to the operating system.

A pull-request (#5) has been submitted, but it was just improving a wrong design decision. Instead, BootUnlock is supposed to get triggered on the "mount" event, so we should let the operating system decide when a volume is attached and launch our script.