Use high resolution tiles

My apologies, this is a rather nice pull request :)

This PR improves the experience of zooming out when `preload` is not set (see #423).

It uses tiles from the next level of zoom level down if the current ones are not available. Google Maps and Leaflet already use this technique. ol3 already has better tile management than both, and this PR adds the remaining missing functionality.

The search is deliberately limited to one zoom level down. Searching further than one zoom level will lead to aliasing artefacts. We'll be safe from aliasing artefacts as long as the ratio of resolutions between zoom levels is <= 2, which is a generally safe assumption.

Please test!