add TrailingSlashHandler

redirects all GET requests with a trailing slash
ensure 'path' never has leading or trailing slash in nbmanager

Don't trust API callers to always get this right.
simplify handler patterns

PATH pattern will now sometimes include leading slash,
but that's okay because nbmanager will strip them.

fewer redirect patterns needed
don't let empty strings introduce extra slashes in url_path_join
properly hook up tornado loggers