add support for provider attribution and logo, as required by Bing. Add an entry...

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>