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.
  Reset is removed because it seems to lock up the WeDo hub. High Power is
  removed and is instead automatically detected based on the current availible
  reported by the USB port that the WeDo hub is connected to.
* 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.