add revised mson spec

@zdne @pksunkara @honzajavorek Please review and merge when ready.

This PR restructures the spec per discussion in https://github.com/apiaryio/mson/pull/4 to progressively introduce functionality and reduce having rules span different sections. Comments in the other PR have been addressed here together with the restructuring and completion of missing definitions (hopefully :)).
I know, but the section is about

```
_Property Name_ → _[Literal Value][]_ | _[Variable Property Name][]_
```

So, I think we should limit the example to that rule only.