Add option to disable url-not-found handling

Add a new option `options.rest.handleUnknownPaths` (true by default)
that controls whether the REST adapter rejects unknown paths with 404.

When `handleUnknownPaths` is false, the REST adapter delegates handling
of unknown paths to the top-level application by calling `next()`.
Merge pull request #135 from drFabio/feature/fix-issue-129

Add option to disable url-not-found handling

Close #135