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preprocessor checks and documentation to the bootloaders giving information about missing SIGNATURE_x defines due to outdated avr-libc versions.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