Replace item_traits class simplifying the code.
Expose self() method from ItemHelper publicly
Rename ItemHelper::self() to data()
Remove unused OSMIUM_WITH_DEBUG
Better implementation for TagList next()/value()
Use 'unsigned char' instead of 'char' for buffer.
Use -pthreads instead of -lpthreads in Makefile
Set has_multiple_object_versions for osmChange XML files