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 :)).
@zdne  Are you saying you would like those bullet points, or change the emphasis? Not sure the subtlety of what you mean by definition vs description comes through or adds much, IMO.