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 :)).
Minor: I would rephrase it slightly to:

```
data structures are described by list Types and/or header-defined and/or combination...
```

- List = description of a data structure
- Header = definition of a data structure