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 :)).
It can be a bit confusing, but I am afraid there aren't much better ways to do it. JSON Schema handles arrays of arbitrary elements and "fixed" arrays (tuples?) the same way as far as I remember.