new feature: "updateInterval"-property in StarlingTileSystem

With this additional "updateInterval"-property it is possible to reduce
the distance for loading tiles and/or their image sizes without creating
"blank spots" which are otherwise filled too late.

This way you can tweak a better tradeoff between distance, tile size and
update interval.
Merge pull request #203 from RazorDE/update-interval

new feature: "updateInterval"-property in StarlingTileSystem