Re-add signature byte defines to the DFU and CDC class bootloaders -- the SIGNATURE_x defines in the AVR device header files only exist in a newer version of avr-libc than is in the Debian repositories, causing build issues on Linux.

Deleted StdDescriptors.c, renamed USB_GetDescriptor() to CALLBACK_USB_GetDescriptor, moved ConfigDescriptor.c/.h from the LUFA/Drivers/USB/Class/ directory to LUFA/Drivers/USB/HighLevel/ in preperation for the new USB class APIs.

All comments in the library, bootloaders, demos and projects have now been spell-checked and spelling mistakes/typos corrected.

Altered DFU bootloader to use the signature byte constants located in the avr-libc part headers, rather than redefining them in Descriptors.h.