Spark Core USB HID Mouse and Keyboard support

Use the following macros in main.h to conditionally compile and provide support for USB Serial/Mouse/Keyboard:

//#define SPARK_USB_SERIAL
//#define SPARK_USB_MOUSE
//#define SPARK_USB_KEYBOARD

USB-CDC Serial is enabled by default.

Pending: new product IDs required for mouse and keyboard from openmoko.org.