Implement function to recursively add a dir to an iso tree.

This commit also to the following changes:
- create_node() on builder never frees the IsoFileSource, it is responsability 
  of the caller to free it.
- Recursive addition options added to IsoImage (not exposed to public API yet)
- create_node() takes care about follow_symlinks
- Added little demo program to test it.

src/builder.h

@@ -48,8 +48,7 @@ struct Iso_Node_Builder
      * created is determined from the type of the file source. Name,
      * permissions and other attributes are taken from source file.
      * 
-     * On sucess, the ref. to src will be owned by node, so you musn't
-     * unref it.
+     * Note that the src is never unref, so you need to free it.
      * 
      * @return
      *    1 on success, < 0 on error