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.

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.

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preprocessor checks and documentation to the bootloaders giving information about missing SIGNATURE_x defines due to outdated avr-libc versions.

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

Corrected CDC class bootloader to fix a few bugs, changed address counter to store x2 addresses for convenience.

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