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 :)).
Good point. I understand it as a `number` as it's [specified in JSON Schema](https://spacetelescope.github.io/understanding-json-schema/reference/numeric.html). Did not read the spec, but on the page linked above they say *exponential notation also works...* with example *2.99792458e8* passing as green. I vote for coherence with this.

And I agree we should definitely specify this better.