Remove artifact in the header.
Rename pathutils.path_string to path_to_string.
Add string_to_path function to pathutils.
Make vyconf.tree.Node.get_child() return self for empty path.
This is probably the most reasonable behaviour, and some internal
operations need special handling for empty path is it causes an error.
Add string_to_path function to vyconf.pathutils.
Add unit tests for pathutils.
Add support for interface definitions that expand other nodes.

<expands path="foo bar"/> attributes in interface definition XML
define what nodes it expands.
To avoid making things messy, <nodeList> is not the root node anymore,
the new root node is <interfaceDefinition>.