Make add and add_attrs chainable.

kml.py

@@ -90,12 +90,14 @@ def add_attrs(self, *args, **kwargs):
         for arg in args:
             self.attrs.update(arg)
         self.attrs.update(kwargs)
+        return self
 
     def add(self, *args, **kwargs):
         """Add children."""
         self.children.extend(list(arg for arg in args if not arg is None))
         for key, value in kwargs.items():
             self.children.append(class_by_name[key](value))
+        return self
 
     def write(self, file):
         """Write self to file."""