Major additions to the API and implementation.  API:  Added
IGPIOPinInputListener and associated event classes.  This allows
whiteboard registration of impls of IGPIOPinInputListener instances that
when registered/discovered will result in the provisioning of a pi4j
GpioPinDigitalInputListener being setup and registered for callbacks.
This implementation is now in org.eclipse.ecf.raspberrypi.gpio.pi4j
bundle.  Also added *Analog interfaces for output and input.  No
implementation of these yet.