Fix capitalisation of GOV.UK.
Define `VERB_json` methods programmatically.
Allow creating custom responses from `get_json`.

For paginated responses, this lets us instantiate a class with more
useful methods on it, without polluting the Response class itself.
We can also pass in a reference to the API client itself, so the
response can fetch subsequent pages.
Allow page traversal from the content API.

Plural responses from the content API now get instantiated as
`ListResponse` objects, which have a method to iterate across any
subsequent pages.
Memoise links between pages of the same list.
Factor out section page stubbing, plus more tests.
Make the ListResponse's delegator clearer.
Fix block style on with_subsequent_pages.

Because we're using the inner blocks for their side effects, we should
be using do...end blocks, rather than braces.
Document the #with_subsequent_pages method.

As rightly suggested by @jamiecobbett.
Explicitly reference the CONTENT_API_ENDPOINT

Probably because of variable load-order, the test failed on my machine.
This change should make it deterministic.
Merge pull request #38 from alphagov/paginated-content-api

Paginated content API