Add imageQueryString option to WebKitCSSResourceCache#getTextureURL

I added this to fix a problem with a `UIWebView`'s caching.

I couldn't find any way of forcing the browser to clear its cache (`NSURLCache` couldn't help) so I added a query string to each texture asset: the app takes a timestamp on start-up and uses it as a query string for its assets. This allows all assets to be "cleared" from cache when the `UIWebView` is reloaded.

If you feel like it's worth merging, please feel free to add it.