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