prepare to release Jellyfish 0.9.0

Enhancements for Jellyfish core

* We no longer use exceptions to control the flow. Use
  `halt(InternalError.new)` instead. However, raising exceptions
  would still work. Just prefer to use `halt` if you would like
  some performance boost.

Incompatible changes

* If you're raising `NotFound` instead of using `forward` in your app,
  it would no longer forward the request but show a 404 page. Always
  use `forward` if you intend to forward the request.