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.

Updated bootloaders to use the new main() function layout and remove any references to the scheduler to keep them in line with the rest of the library.

Fixed incorrect ENDPOINT_EPNUM_MASK mask preventing endpoints above EP3 from being selected (thanks to Jonathan Oakley).

Removed STREAM_CALLBACK() macro - callbacks now use regular function definitions to clarify user code.

Removed DESCRIPTOR_COMPARATOR() macro - comparators should now use regular function definitions to clarify user code.

Fixed Endpoint_Write_Control_* functions writing more data than expected by the host, causing it to panic (thanks to Johannes Raschke).

Added preprocessor checks and documentation to the bootloaders giving information about missing SIGNATURE_x defines due to outdated avr-libc versions.

Makefiles and library modified to add a new F_CLOCK constant to give the unprescaled master input clock frequency, so that the correct PLL mask can be determined even when the CPU (F_CPU) clock rate is prescaled outside the normal input range of the PLL.

Started to clean up the AVRISP Programmer project code, donated by Opendous Inc.