Add a standard QGeoTiledMapData subclass, which handles equirectangular projection

It seems to be helpful to provide a QGeoTiledMapData subclass, which handles equirectangular projection. If it is provided, plugin writers which want to use such projection does not take care about internal coordinate translations.

I've attached such a class and a test case for this class.

Also the documentation on subclassing QGeoTiledMapData should be improved. Because the worldReferenceSize() values are not that intuitive to understand.

By the way, I've also created an plugin which uses the NASA bluemarble map (day and night) in offline mode as a map. Are you interested in providing this equirectangular map plugin too?