Add 'control' module to the list exported by usb package.

'control' was not in the __all__ field for the 'usb' package. This means that
  from usb import *
  control.XXX
would fail to find the module 'control'.
Merge pull request #50 from callpraths/add_control_to_all

Add 'control' module to the list exported by usb package.