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 always hated examples with *foo* and *bar*, because they aren't from the real world and they don't actually illustrate anything - sometimes they even confused me more.

The *T* is like *foobar* from my POW - isn't illustrative.