Add gds-api-adapters gem

It includes an ExternalLinkTracker adapter
Add ExternalLinkRegisterer

Uses an API adapter to register external links with the external link
tracker.
Refactor IndexingTask

Extract lots of methods to make it easier to follow and extend.
Take indexer in IndexingTask constructor

It makes more sense there than in the run method
Register external links as part of IndexingTask
Don't provide defaults for IndexingTask options

Instead of building a default Indexer and ExternalLinkRegisterer just
don't run anything if they aren't provided.

This allows us to keep the behaviour of the rummager:index rake task the
same and swap out for a new rake task when we're happy it's behaving
properly.
Add deploy_links rake task

Adds links to the rummager index and the external link tracker and
removes old links from the rummager index.
Merge pull request #22 from alphagov/register-urls-with-external-link-tracker

Register urls with external link tracker