Add test for URLPath
Fix URLPath.

Move where we modify prepath and postpath so that it's done before
the handler is called.  This means request.URLPath() inside the handler
will point to the route of the handler.
Add test for URLPath at root, and URLPath in a traersed resource.
Merge pull request #15 from twisted/fix-urlpath

Fix urlpath
Merge branch 'master' into finished-requests

Conflicts:
	klein/test_resource.py