WL-7 Allow hierarchy to claim /site URLs.

We have our own /site handler and when we re-claim the /site URLs it stops the error handling working as the handlers get de-registered for GC purposes. So we create a special “internal” instance to allow the error handlers to carry on working.
Merge pull request #12 from ox-it/WL-7

WL-7 Allow hierarchy to claim /site URLs.