Wedo Hub support

* Add USB HID driver for WeDo hub.
* Add wedo bus in sysfs
* Add support for Wedo (Power Functions) motors connected to a WeDo hub.
* Add support for Wedo Motion and Tilt sensors connected to a WeDo hub.

Other changes:

* Includes update to the dc motor ops to use direction instead of polarity to
  set the motor rotation
* Rename dc-motor class duty_cycle to duty_cycle_sp.
* Add new duty_cycle attribute that returns current duty cycle.