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. 
Prefer single-quoted strings when you don't need string interpolation or special symbols.