* Move `is_active` to `GPIODevice`; it's equally applicable to inputs
and outputs so there's no point having it just in inputs
* Flip the pull-up status for `MotionSensor` (it was backwards leading
to reversed readings from the sensor)
* Add a `threshold` to `MotionSensor` (optional), and `value` (similar
to `LightSensor`)
* Also expose `pull_up` as a simple bool property
* Rejig `LightSensor` so it also derives from `InputDevice` (it inherits
enough to make it worthwhile) and so that its API is similar to
`MotionSensor` (a `value` property with a `*_detected` property, and a
background threaded queue which constantly monitors values)
The motion sensor queue doesn't shut down properly at script end at the
moment and prevents the interpreter shutting down. This is because it's
a non-daemon thread so `__del__` never gets run and so on.
This is a bit of a major PR - I can split it up if you want. Firstly it
makes a common base class called `GPIODevice` for both `InputDevice` and
`OutputDevice`. This just takes care of the read-only pin stuff. Next it
makes a `GPIOThread` class that ensures its a daemon thread, and which
also ensures proper cleanup on shutdown.
Finally, it fixes `MotionSensor` to use the new `GPIOThread` class
(tested this time! Works nicely) and adds the `blink` method to the
`LED` class (which also works nicely this time).
