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/image.c

@@ -77,6 +77,16 @@ int iso_image_new(const char *name, IsoImage **image)
         return res;
     }
     img->refcount = 1;
+    img->recOpts = calloc(1,sizeof(IsoImageRecOpts));
+    if (img->recOpts == NULL) {
+        libiso_msgs_destroy(&img->messenger, 0);
+        iso_node_builder_unref(img->builder);
+        iso_filesystem_unref(img->fs);
+        iso_node_unref((IsoNode*)img->root);
+        free(img);
+        return ISO_MEM_ERROR;
+    }
+    
     if (name != NULL) {
         img->volset_id = strdup(name);
         img->volume_id = strdup(name);