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/libisofs.h

@@ -638,6 +638,22 @@ int iso_tree_add_new_special(IsoDir *parent, const char *name, mode_t mode,
 int iso_tree_add_node(IsoImage *image, IsoDir *parent, const char *path, 
                       IsoNode **node);
 
+/**
+ * Add the contents of a dir to a given directory of the iso tree.
+ * 
+ * TODO comment Builder and Filesystem related issues when exposing both
+ * 
+ * @param image
+ *      TODO expose dir rec options and explain that here
+ * @param parent
+ *      Directory on the image tree where to add the contents of the dir
+ * @param dir
+ *      Path to a dir in the filesystem
+ * @return 
+ *     number of nodes in parent if success, < 0 otherwise
+ */
+int iso_tree_add_dir_rec(IsoImage *image, IsoDir *parent, const char *dir);
+
 /**
  * Locate a node by its path on image.
  *