Access project from todo_list

This PR is to be able to access project and project_id from todo list:

```ruby
> Basecamp::TodoList.find(list_id).project_id
=> 1234
> Basecamp::TodoList.find(list_id).project
=> #<Basecamp::Project:0x007fb893d615e8...
```

Ref: https://github.com/anibalcucco/basecamp-wrapper/issues/29

@sprestel can you test this branch in your project and confirm is working?