Initial support for low level ECMA-119 tree.

This only adds the tree creation functions, no sorting and name mangling 
yet. Only plain ECMA-119 supported for now, nor RR, relaxed 
restrictions, etc..

This also adds a little test program.

demo/tree.c

@@ -1,6 +1,5 @@
 /*
- * Little program that reads an existing ISO image and prints its
- * contents to stdout.
+ * Little program that import a directory and prints the resulting iso tree.
  */
 
 #include "libisofs.h"