fix(nickel): don't force default_router to be last middleware

Prior to this change, when *not* bootstrapping
the router manually, there was no way to have
other middleware after the default router.

This changes the code to use one router per
registered route which works around the
limitation.

Kudos to @reem for the suggestion.

Relates to #59
Relates to #60
fix(example): return proper 404 for unmatched routes

Relates to #60