Add BingTileLayer to get map tiles from Bing

Based off of https://github.com/osmdroid/osmdroid/blob/master/osmdroid-third-party/src/main/java/org/osmdroid/tileprovider/tilesource/bing/imagerymetadata/ImageryMetaDataResource.java
Merge pull request #519 from kjeremy/bing

Add BingTileLayer to get map tiles from Bing