libpixyusb SWIG module development

+ Added OS identification in build script for libpixyusb SWIG python module.
  This fixes USB device intialization issue that is specific to Linux.
+ Added a debug library "debug.h" that is activated when the C preprocessor symbol
  "DEBUG" is defined.
+ Modified build_libpixyusb.sh and build_libpixyusb_swig.sh scripts to support
  building in debug mode, which builds with debugging symbols and enables
  logging.

  To build with debug, just add a "debug" as a parameter. For example:

  > build_libpixyusb.sh debug