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 :)).
So, I think we should leave it because there are some sections where a term cannot be linked to because it is just a locally defined syntactic element. Thus, there will always be some that are never going to be links.