add post handling to library

[erikh]

This provides POST request handling with reqwest::Body as input (to avoid prescribing a specific serialization method). Honestly, since this is so simple to bootstrap, a better patch would be to change the interface to just accept a reqwest client that was already built and work with that instead of fixating on specific methods.

This patch is working for me in a private project, I don't expect to change my needs here and I will just run with the fork if this is uninteresting. I do hope it helps, however. Thanks.

[erikh]

I realize that I trampled right over the literal thing I wanted to make this patch, so I added a helper for PUT too that works exactly like POST, corrected the docs, and I hope it's convenient, at least.

If not I'll just toss this, thanks.