Added iOS 6.1 support.

Hi Sam,

I updated the latest SVProgressHUD to support iOS 6.1. The changes prevent the crashes others have reported when using SVProgessHUD on iOS 6.1. You can test the changes by running the demo app on iOS 6.1. Feel free to merge in the changes if you like. And thanks for your awesome work on SVProgressHUD.

-KIRBY
Thanks @kirbyt - solved a problem I was having with something completely unrelated!