Added parameter directions to function parameter documentation.

Added new experimental FAST_STREAM_FUNCTIONS compile time option to speed up stream transfers at the expense of a higher FLASH consumption (needs testing to verify improved throughput).

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.