Added a customSubview property to support SBTableAlert on iOS 7

[mindbrix]

Hi Tom. I have added a customSubview property to support SBTableAlert, which was broken by iOS 7 as it no longer allows subviews on UIAlertView. My solution was to replaced UIAlertView in SBTableAlert with TSAlertView. It worked.

Now all we need to fix is the old iOS 6 style. ;-)

My SBTableAlert fork is at https://github.com/mindbrix/SBTableAlert

Thanks for the great code, Nigel.