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.

.bzrignore

@@ -26,3 +26,4 @@ test/iso
 test/test
 demo/lsl
 demo/cat
+demo/tree