Add Pavel Shramov's Bing tile layer plugin for Leaflet

Retrieved on Wed Aug 14 22:03:32 CEST 2013 from
https://raw.github.com/shramov/leaflet-plugins/master/layer/tile/Bing.js

Copyright (c) 2011-2012, Pavel Shramov
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Add Bing Maps key settings
Set maximum zoom level to 19
Show Bing Maps satellite imagery