Remove secrets.yml

This is only used for cookie signing, which this API doesn't need.
Update README around error format

This brings the documentation in line with the current implementation.
Merge pull request #22 from alphagov/remove-unnecessary-secrets-file

Remove secrets.yml
Merge pull request #23 from alphagov/fix-errors-documentation

Update README around error format
Delete unnecessary .keep file
Add sanitize gem to detect dangerous HTML tags
Assert manual text fields have no HTML
Document text sanitisation in README
Extract check for HTML into module
Fix test helpers for sections

Before this change, the section helper methods took parameters
but then just ignored them.
Assert section text fields have no HTML
Refactor WithoutHtmlInTextFields into a validator

This makes the code conform closer to rails conventions.
Change key in `StructuredData#string_fields`

The method now returns a list of [ path, value ]
Move JSON schema validation into a Validator