When calling 'sub_node' on a node with no sub nodes, the Xml_node would
interpret the characters after the current node while searching for sub
nodes. The patch adds a sanity check that lets the 'sub_node' function
throw an exception when called on a node with no sub nodes.
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082
