Added a CountdownEvent class to pelix.utilities

An Event which is set once its internal counter reaches 0.
Enhanced thread pool's FutureResult to support a callback method

Instead of "actively" waiting for the result to come, a method can be
set via FutureResult.set_callback(). This method will be called once the
execution is done or in case of error.
An extra parameter can be given to the called back method.