shiftOut should be a method on board instances

[rmurphey]

I've been looking at http://www.arduino.cc/playground/Main/LedControl, which is a library for using a MAX7219 chip to control an LED matrix or a set of seven-segment displays. It uses the shiftOut functionality that I originally saw in shift register examples. Presently, johnny five implements the shiftOut functionality in the ShiftRegister API, but it doesn't provide it as a separate method. I think that this should actually be a method on board instances, rather than falling under the ShiftRegister API, because I'm understanding now that it has broader utility.

Pull request coming shortly, but more for discussion than necessarily to be landed.