Prevent the navigator from making an unnecessary Ajax request.

Previously, when showNavigator was set to true when creating the viewer, the navigator would unnecessarily load and parse the tile source, even though a fully parsed object already exists.

PS: The README states the changelog should be updated in the corresponding pull request, but from the discussion in #90, it looks like you want to do this yourself to prevent merge conflicts. If that's correct, I guess the README should be updated.