Fixed mounting Derby app to an Express app using route other then '/'

Without this fix routing will work only on server side, because request
would be handled by Express first before passing it to `tracks` router.

Client side routing will fail because `tracks` router will receive
prefixed URL for handling, e.g. it will receive '/derby-app/home' while
knowing only about '/home' route.

P.S. change is done over v0.1.12 tag because this is the version compatible with Derby 0.3.15. If it fails to merge, I can redo over master.