Problem: The gsl file to build zyre build system files are out of date

Solution: Remove local zyre model and use zproject

Side-Effects from using zproject:

   * Needed to move the api from zyre.h to zyre_api
   * Only distribute none private header files
   * Moved zre_msg.h into src as this should be private
   * Wrapped private classes in zyre_classes.h which is know generated
   * Private constanst in zyre_classes.h are also generated, see
   project.xml.