Make sure all sections are marked as normative/informative

Also updated the conformance section to include the following statement which is found in most W3C specs:

     As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

This addresses the problem that ReSpec doesn't mark normative sections but just non-normative sections.

This addresses #224.