Datapoint class misses created_at field

[matkoniecz]

https://github.com/beeminder/beeminder-gem/blob/master/lib/beeminder/goals.rb#L228

[matkoniecz]

though looking at commit dates ( https://github.com/beeminder/beeminder-gem/commits/master/ ) I guess I should rewrite my code to use REST API directly?