Fix doxygen documentation - doxygen was getting confused on the variable-length ATTR_NON_NULL_PTR_ARG() macro used on  function prototypes and was generating many return types for several functions in the documentation (thanks to David Lyons).

Make Low Level host demos use void pointers for the configuration descriptor, to prevent warnings when passed to the altered configuration descriptor parsing routines.

Added preprocessor checks to give a human readable error when the class drivers are used when the incompatible NO_STREAM_CALLBACKS compile time option is used.

Split out common defines/types from class drivers into a seperate common class driver directory.

Make central USB Class driver dispatch headers, used for both device and host modes.

Move new Class Driver powered demos to a new ClassDriver subdirectory, re-add old low level demos to a LowLevel subdirectory.

Moved out each demos' functionality library files (e.g. Ring Buffer library) to /Lib directories for a better directory structure.

Renamed Serial_Stream driver to SerialStream to remain consistent with the rest of the library's naming scheme for files.

Seperated out OTG, Device and Host mode demos into seperate folders for clarity. Adjusted makefiles so that the path to the LUFA library can be set in one place.