Create DocViolation for modules and classes with methods

Fixes #37
Based on #10

We currently create a `DocViolation` for every class that's missing a comment, even if it's empty. For example, this currently creates a violation:

```ruby
class NotAllowed
end
```
This PR changes `DocCheck` so that classes and modules without methods do not create violations:

```ruby
module NoDocsNeeded
  class NoDocsNeededEither; end
end
```

And classes and modules with methods do create violations:

```ruby
module MyAwesomeGemDoesntNeedDocs
  # Need docs for this  
  class CLI
    def run; end  
  end
end

# Need docs for this
module Options
  def self.default
  end
end
```