2007-05-12	Timo Bingmann
	* btree.h: Fixed segfault in print() because of non-existing root.
		   Fixed segfault in end() when the tree is totally empty.
		   Added BTREE_FRIENDS macro so that wxBTreeDemo can
		   access private members.
		   Changing print function to output to a user-given
		   std::ostream