Made borderless iOS7 style popover more like the real iOS7 popover

Hello,
Thanks for this awesome project and I can show popover on iPhone now!
I just made a few minor changes on how this is displayed, to make it mimic iOS7 popover slightly better:
1) I added a background dimming (in iOS 7/8 the default popover does dim the window background)
2) when .border is set to false, the viewController.view's size is matching the contentSize now. So you can set the contentSize to the exact size of your view. (before this change, the view will get cut off and the popover will be smaller than the contentSize you set, with transparent margins on each side)
3) the arrow looks balanced in .border = false mode now.

Most of the changes (except (1)) are completely limited to when the border is set to false. 

Thanks,
Bao