Move StdRequestType.h, StreamCallbacks.h, USBMode.h from the LowLevel USB driver directory to the HighLevel USB driver directory, where they are more suited.

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

Fixed documentation typoes and preprocessor checks relating to misspellings of the USE_RAM_DESCRIPTORS token (thanks to Ian Gregg).