Add key operations to Traverson

[gregturn]

It would be useful to have get, post, put, and delete, as well as an open-ended equivalent to exchange.

[HPacquee]

#225 Has some discussion about this.

Don't know if this is useful, but at my client we've added this functionality for ourselves some years back. I've added how we do it in a PR ( #637). Everything bar the exchange