Proposed update to API spec for JSEP

I've been working on a proposal for how to change the API spec to be inline with JSEP. I have attached the proposed change to this email along with a diff. These are far from perfect and some more would need to be done but ...  I think they get the main structure and are enough to start talking about if we want to use this a starting point for the Editor draft. 

Have a look at section 4.1 and review it from the point of view of, is this close enough to start using as the editor draft. 

Thanks, Cullen