Tests for tiled image map view controller + bugs.

Fixed downloading of tiles that may happen multiple times. In a slow network tiles will take a long time to download, but you may get lots of repaint events. This will cause the queue to get more downloads for the same tiles already being downloaded. We now hold on to a map of URLs to downloads and make sure to only have a single download operation per url running.

Added tests for the tiled image view controller. Notice a missing tile in the snapshot image, this seems to be a bug in iOS (@orta, can you comment with some links, please?).