Convert WeDo driver to new lego bus

* The WeDo hub itself is now a lego-sensor. The status and voltage attributes
  are converted to values and the other attributes are converted to commands.
* The WeDo ports are now lego-port devices.
* Sensor, motor and led devices remain the same.
* Fixed sleeping (registering/unregistering sensor/motor/led devices) in
  interrupt context.
* Also includes some code cleanup.