Validate the format of Section IDs in JSON

By establishing that Section IDs follow some rules, we can pre-empt errors when
constructing the data we send to Publishing API.
Merge pull request #48 from alphagov/validate_section_ids

Validate the format of Section IDs in JSON
Slugs in request URLs must be lowercase

The GOV.UK slugs/URLs will be lowercase, so let's enforce it to avoid a
situation where they send it uppercase but we neglect to downcase it.

We will still need to remember to downcase Section IDs in breadcrumbs and
child_section_groups before we make them into slugs as part of base_paths.
Ensure that base_paths are always lowercase

GOV.UK URLs are case sensitive and the styleguide says they must be lowercase:
https://insidegovuk.blog.gov.uk/url-standards-for-gov-uk/
Add rules to README
Slugs in specs should conform to the styleguide

Practice what you preach.
Merge pull request #49 from alphagov/validate_slugs

Validate slugs and ensure they are lowercase
Error with a 415 code if the Content-Type is not application/json

- This uses a `before_filter` in ApplicationController to check
  Content-Type, leaving `respond_to` to deal with the Accept header.