[libxml++] My proposed patches

[Jim G. <gar...@ca...>]

The first patch adds cobj() to Document.  The second one adds the
function Document::create_root_node_by_import.  I decided to leave the
long name and not overload create_root_node for the sake of clarity; in
the same manner, Node::import_node and Node::add_child are two
differently named functions with analogous functionality.

If you give me the word, I'll go ahead and commit it to the module, or
feel free to do it on your own.

Thanks,
Jim