Ability to use it to implement WOT discovery

[arpad-m]

I want to use this library to implement the WOT discovery spec, but I'm running into some problems. I'm opening this as meta issue so that I can link the other issues from here.

The implementation lives here and I will likely need changes to webthing-rust. I want to upstream as much as possible.

• Support non-observable non-POD properties #73
• Anyone can delete actions, edit properties, etc #74