dropping legacy doorkeeper_for and welcome doorkeeper_authorize!

Details has discussed at PR #434

- dropped legacy `doorkeeper_for`
- new `doorkeeper_authorize!` method, let Rails' `before_filter`(perhaps `prepend_before_filter` is better) handle cases which resource need to be protect
- `includes_scope?` would force `to_s` for every given scope(means upper invoker no need to care about data type)
- clean up no need test cases
Because `authorize_doorkeeper` handle render(if error), that not compatible with Grape or Sinatra.
`doorkeeper_token` and `valid_doorkeeper_token?` is not rails-specific