Make the top things poller work with the new relative time offset API

Since the API now directly supports time strings like `"-24h"` and `"30d"`, we can just pass those values straight to the API without any fancy conversion stuff.

This will be released as a new minor version, so old things needn't worry.

See https://github.com/MassRelevance/massrel-js-backbone/pull/9 for the accompanying PR for `massrel-js-backbone`.