add .shutdown() method to GpioController

[savageautomate]

Add a .shutdown() method to the GpioController interface and implementation to forcefully cleanup and shutdown all GPIO related threads.

[savageautomate]

Added support for this feature in the latest 0.0.5-SNAPSHOT (2013-02-09) build.

There is also a new ExecutorServiceFactory implementation that allow users to provide their own executor service factory where you users can create the executors and manage own thread pool. Users can assign their own ExecutorServiceFactory implementation via the GpioController.

See: https://github.com/Pi4J/pi4j/tree/develop/pi4j-core/src/main/java/com/pi4j/concurrent for more details.

Example code files all updates to include this new GpioController.shutdown() method implementation.
See: https://github.com/Pi4J/pi4j/blob/develop/pi4j-example/src/main/java/ControlGpioExample.java#L81