Create a common section for responses

Previously, the two sections differed in style and missed entries that were
included in the other.

We may want to revisit this if/when we have eg DELETE routes.
Merge pull request #10 from alphagov/restructure_responses_in_readme

Create a common section for responses
Add 'json-schema' gem for validation
ManualsController validates incoming JSON

The `json-schema` gem is used for validation.
Move manual schema to separate file
Validate incoming JSON in the SectionsController

- We don't need to specify `require 'json'` - the tests still pass
  without it.
Merge pull request #12 from alphagov/validation-of-manual-title

Validation of manual title
Add examples of JSON

These are the manual and some sections of the scraped Employment Income Manual
from manuals-frontend, modified to fit our initial schema.

- EIM00100.json is the 'About' section, with content in the body and no child sections
- EIM77010.json is an appendix, with more content in the body and no child sections