Added a unique_key method for Namespace objects which is compatible with the unique_key method for Node objects. Now we don't have to check the type of object returned by xpath methods before we can call unique_key. Tests and documentation included.
The unique_key method simply returns a concatenation of the namespace prefix and URI, with a vertical bar separating them. This is consistent with the behavior of _isEqual, which checks for both prefix and URI equality between two namespaces.
Note that unique_key.t now relies on the Namespace constructor working, which isn't tested anywhere else. Might be good for the TODO list.