revert NotFoundResource.

apologies for how long this took...! this should close #155 and restore webmachine's 404 handling to something way more normal. it will also reintroduce this bug described in this test though, which seems less critical than what's going on in #155: https://github.com/seancribbs/webmachine-ruby/commit/9ab516a02861e7e12715a55b571855748f4962cc

unfortunately, im pretty sure time won't let me fix that.