Redocumented all device demos, now that they have changed over to the new USB class drivers.

Added C linkage to class drivers for C++ support.

Added prefixes to most of the class driver constants to prevent name clashes.

USB_IsConnected is now cleared before the USB_Disconnect() event is fired in response to VBUS being removed.

Minor documentation improvements.

Fixed bug in RNDISEthernet and DualCDC demos not using the correct USB_ControlRequest structure for control request data.

Fixed documentation showing incorrect USB mode support on the supported AVRs list.

Changed HWB board driver to Buttons driver, to allow for the support of future boards with more than one mounted GPIO button.