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 :)).
Even though this is just an introduction to this section, the two items before this "or, equivalently" section (which were Header-defined and List-defined) had links to the specific area in the spec (Named Type and Member Type). Should this section also do this? 

This is a minor thing, I know, but I did personally get this section and wonder what "Person" meant. Was it referring to the Person above or just an example.