Add properties to schema

[jasollien]

No description provided.

[jasollien]

@pasi-paasiala, @ykulbak , @GeorgDangl: Could you have a look at this, and give some feedback. I have tried making it based on your ideas.

[ykulbak]

@jasollien I think it's a good start. I think we should also allow extension schema information regarding the allowed values for some properties and possibly topic authorisation information regarding changing the values of properties.

I've spent a few minutes reviewing and I need more time to think about this.

[pasi-paasiala]

I think we also need to have enumeration properties and be able to specify physical quantity (like length) to numerical properties.