Added basic driver example use code to the library documentation.

Made the USARTStream global public and documented in the SerialStream module, allowing for the serial USART stream to be accessed via its handle rather than via the implicit stdout and stdin streams.

LUFA/Drivers/Board/Buttons.h

@@ -60,6 +60,21 @@
  *
  *  For possible BOARD makefile values, see \ref Group_BoardTypes.
  *
+ *  <b>Example Usage:</b>
+ *  \code
+ *      // Initialise the button driver before first use
+ *      Buttons_Init();
+ *
+ *      printf("Waiting for button press...\r\n");
+ *
+ *      // Loop until a board button has been pressed
+ *      uint8_t ButtonPress;
+ *      while (!(ButtonPress = Buttons_GetStatus())) {};
+ *
+ *      // Display which button was pressed (assuming two board buttons)
+ *      printf("Button pressed: %s\r\n", (ButtonPress == BUTTONS_BUTTON1) ? "Button 1" : "Button 2");
+ *  \endcode
+ * 
  *  @{
  */