[ios] Set Admob banner to appear at bottom of screen

Set Admob banner to appear at bottom of screen

Set in Portrait view:

        bannerView_.frame = CGRectMake(screenWidth/2.0 - bannerView_.frame.size.width/2.0, screenHeight - bannerView_.frame.size.height,
                                       bannerView_.frame.size.width,bannerView_.frame.size.height);

Set in Landscape view:

        bannerView_.frame = CGRectMake(screenHeight/2.0 - bannerView_.frame.size.width/2.0, screenWidth - bannerView_.frame.size.height,
                                       bannerView_.frame.size.width,bannerView_.frame.size.height);