Don't load the tile source again when creating a navigator.

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.