Add host mode USB Class driver stubs, add beginnings of a CDC host class driver.

Split out common defines/types from class drivers into a seperate common class driver directory.

Make central USB Class driver dispatch headers, used for both device and host modes.

LUFA/Drivers/USB/Class/Device/Audio.c

@@ -28,6 +28,9 @@
   this software.
 */
 
+#include "../../HighLevel/USBMode.h"
+#if defined(USB_CAN_BE_DEVICE)
+
 #include "Audio.h"
 
 void Audio_Device_ProcessControlPacket(USB_ClassInfo_Audio_t* AudioInterfaceInfo)
@@ -157,3 +160,5 @@ bool Audio_Device_IsReadyForNextSample(USB_ClassInfo_Audio_t* AudioInterfaceInfo
 	Endpoint_SelectEndpoint(AudioInterfaceInfo->DataINEndpointNumber);
 	return Endpoint_IsINReady();
 }
+
+#endif