ISSUE-8
Add showPopover completion
UIView
public func showPopover(sourceView: UIView, sourceRect: CGRect?, completion: ShowPopoverCompletion?)
public func showPopover(barButtonItem: UIBarButtonItem, completion: ShowPopoverCompletion?)
UIViewController
public func showPopover(sourceView: UIView, sourceRect: CGRect?, completion: ShowPopoverCompletion?)
public func showPopover(withNavigationController sourceView: UIView, sourceRect: CGRect?, completion: ShowPopoverCompletion?)
public func showPopover(barButtonItem: UIBarButtonItem, completion: ShowPopoverCompletion?)
public func showPopover(withNavigationController barButtonItem: UIBarButtonItem, completion: ShowPopoverCompletion?)