Add an HTML-parsing search client.

As part of this, rename SearchClient to JSONSearchClient to make it
explicit what it's doing.
Change HTML selector to avoid recommended links.

This ties us more closely to the element hierarchy in the results, but
that may be no bad thing, as it's more likely to give us obviously wrong
results (all failures) if the structure changes.
Allow selection between clients based on format.

We can differentiate between API responses and internal Rummager responses
based on their structure, but testing against the HTML front-end requires a
different URL structure, so we need to switch explicitly.
Add frontend use cases to the README.
Add the download step; reformat the test steps.
Eliminate stray `puts`.
Merge pull request #6 from alphagov/html-adapter

HTML adapter