Use webView.loading instead of manually counting

`MPAdBrowserController` was counting up and down to track whether or not the view was still loading - however, `UIWebView` offers the `loading` property to indicate if the web view is still loading, so instead of manually trying to balance "start" and "end" load calls, just check if it's still loading in the `webViewDidFinishLoad:` delegate call before adjusting the UI to match the loading state.