Creating in-order keys

Hey there.

etcd allows to create in [order keys](https://github.com/coreos/etcd/blob/master/Documentation/api.md#atomically-creating-in-order-keys) using a post method.

I needed this feature, so I extended the `set` method to take an optional `method` parameter as a option

Do you prefer to have a separate method to create in-order keys or do you like the approach with the `set` function?
Before I continue to write code, I would discuss with you what method you prefer.