Custom handler for resource 404

[valerauko]

Issue

#89

Changes

Added optional :not-found-handler param to create-resource-handler.

It defaults to the same constantly as create-default-handler does.

Tests

All current clj tests are green.

I didn't add tests for the new parameter -- should I? If so, to all the test branches (inside/outside router, root/path)?

[ikitommi]

Thanks!