Fix TODOs in the doorkeeper_for class

This just fixes these two TODOs:

* https://github.com/doorkeeper-gem/doorkeeper/blob/master/lib/doorkeeper/doorkeeper_for.rb#L14
* https://github.com/doorkeeper-gem/doorkeeper/blob/master/lib/doorkeeper/doorkeeper_for.rb#L38

I added an `#acceptable?` method to AccessToken and a `#includes_scope?` method to the Scopes Module. `doorkeeper_for` now tests if a token is acceptable by passing the required scopes. 
We should use `Doorkeeper::OAuth::Helpers::ScopeChecker.valid?` instead of creating the new method. I'm sure there's more places in the codebase to DRY up this functionality.