add support for provider attribution and logo, as required by Bing. Add an entry to imagery.xml like this:
<set>
<name>Bing</name>
<url>http://ecn.t0.tiles.virtualearth.net/tiles/a$quadkey.jpeg?g=587&mkt=en-gb&n=z</url>
<scheme>microsoft</scheme>
<sourcetag>Bing</sourcetag>
<attribution_url>http://dev.virtualearth.net/REST/v1/Imagery/Metadata/Aerial/0,0?zl=1&mapVersion=v1&key=Arzdiw4nlOJzRwOz__qailc8NiR31Tt51dN2D7cm57NrnceZnCpgOkmJhNpGoppU&include=ImageryProviders&output=xml</attribution_url>
<logo>bing_maps.png</logo>
<logo_url>http://www.bing.com/maps</logo_url>
</set>