Add methods to HierarchicalConfiguration to obtain sub configurations for child elements

So far, there is no direct way for a hierarchical configuration to obtain all elements contained in a given section (i.e. the child elements of a specific key). This can be achieved by operating on a ConfigurationNode object directly, but this is a low-level API, and for further processing of the element's content it might be necessary to create a SubNodeConfiguration around the node again.

Therefore, it makes sense to extend the API of HierarchicalConfiguration. A method can be added - similar to configurationsAt() - which expects a key, locates the corresponding node, and then creates SubNodeConfiguration objects for all direct children of this node.

It would also be useful to provide a way to query a configuration for the name of its root element. This makes it easier to deal with dynamic configuration structures.