Implement attribute inheritance

This PR aims to implement inheritance for attributes.

Currently this is not possible:

```ruby
class Book
  include Lotus::Entity
  self.attributes = :title, :author
end

class NonFictionBook < Book
  self.attributes = :price
end
```
For the `NonFictionBook` Entitiy only `price` is available as  attribute.
With this PR merged in this is possible because we are saving the inherited attributes as class instance variable via the inherit hook.
Note that this contains some metaprogramming I wish we could remove and therefore have a clearer and easier to understand codebase.

This closes #98
Hey we could use .attribute here, but I think when using the more verbose way it's clearer we are checking the internal state of our object and not relying on a public accessor.