ActiveRecord association helper

Add convenience method to associate ActiveRecord model ids to the Stretchy::Record

It may be that using the normal association methods will work fine. 

```ruby
class Post < Stretchy::Record
  # where User is an ActiveRecord model
  belongs_to :user, class: User
  # where Usage is a Stretchy::Record 
  belongs_to :usage 
end
```

```ruby
post.user #User
```