update to UIActivityViewController

Hey Sam, I found your webviewcontroller very helpful in an app project I was working on. I updated the pageActionSheet to the iOS 6 UIActivityViewController, which looks/feels a bit nicer (I didn't remove the reference to the actionSheet, but just altered what the actionButtonClicked method presents). I also added two UIActivity objects for the ActivityVC, to allow users to open in safari and chrome. I grabbed them from these places: 

https://github.com/alextrob/ARChromeActivity
https://github.com/davbeck/TUSafariActivity

Thanks!