Adding base button press event handler

* Version 1.0.16:
* Date: 14th May 2013
* Compatible/tested with Spokes SDK version(s): 2.8.24304.0
* Changed by: Lewis Collins
*   Changes:
*     - Adding base button press event handler so apps can know when
base button
*       is pressed.
*     - Changing headset button press event registration to avoid
duplicate headset
*       talk button pressed events