Add maxZoom option to BingMaps

This is to support the use-case where stretched tiles are displayed instead of BingMaps "no photos at this zoom level" tiles beyond zoom level 19.

See this [request](https://groups.google.com/d/msg/ol3-dev/WXIFvf_w1-4/BZ7qX-Xcx1YJ) on the mailing list for example.