Support the 'start' parameter to paginate the latest feed

The first part of [this story](https://www.pivotaltracker.com/story/show/79185504).

This adds support for paginating the latest changes feed in the Collections API. 

A subsequent page can be requested by providing the `start` parameter as an integer of the number of documents to offset. This is then used to build the request to Rummager in the `LatestChanges` model.

Additionally, we expose the `start` and `total` values from the Rummager response as keys in the `details` hash of the API response (as `documents_start` and `documents_total` respectively).

_Note: Due to the complexity of running this app in the development environment (due to requiring data for the content store, content API and search indexes), I've not tested this fully locally, but the tests all pass for me._
frontend also checks for negative (https://github.com/alphagov/frontend/blob/master/app/controllers/search_controller.rb#L78 - `valid_start_param` in SearchController). I'd suggest copying the logic from there.

If implementing "count", the logic for that would also be good to copy: https://github.com/alphagov/frontend/blob/master/app/controllers/search_controller.rb#L78