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
Instead using instance_variable_get, you can use .attribute, it's less verbose, what do you think?