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. 
Sorry it took me a while to come back to these (work stuff), but I was looking at the `ScopeChecker.valid?` method and it looks like doesn't quite have the right behaviour for this check. 

It does an `.all?` match between the server_scopes and the token's scopes, when I think an `.any?` match is actually correct. For instance if a controller method has scopes declared as `scopes: [:public, :admin]` this would check fail if the supplied token only has the `:admin` scope. 

I'm basing my definition of what seems correct on how Doorkeeper currently behaves, but it doesn't look like that's something actually defined in the OAuth spec.