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 :)).
> Further, such an `object` type structure MUST NOT contain any other properties.

So by default MSON objects are `additionalProperties: true`, but when fixed, they're (above other implications) `additionalProperties: false`.