Various improvements to the Graph API

The Graph API needs a few improvements, including:
1) add a request to verify the existance of a node (basically, this checks for existence and fetches the actual location)
2) add a way to copy a node by specifying the path of the desired location (in addition to the current mechanism of specifying the parent and inferring the name of the new node from the original). This could be accomplished by adding the desired name to the CopyBranchRequest, and then updating all of the RequestProcessor implementations to use this optional information. This may affect a number of classes.
3) make NamespaceRegistry.Namespace extend Comparable<Namespace> so that namespaces can be sorted naturally
4) improve the efficiency of the AbstractPath.equals(...) implementation, which is not efficiently iterating up the Path.Segment instances.