fix for when_pressed and uiactionsheet

without checking for recognizer.state==UIGestureRecognizerStateBegan, if you call a new UIActionSheet from inside a when_pressed handler two identical `UIActionSheet`s are instantiated and displayed:

```
view.when_pressed do
    UIActionSheet.alert("Log out?", buttons: ['Cancel', "Log out"]) do |pressed|
      if pressed=="Log out"
        #do something
      end
    end        
end
```

this update fixes `when_pressed` to work as expected. that is, to only show a single UIActionSheet when the above code is executed.


see also: http://stackoverflow.com/questions/15363311/uigesturerecognizerstatebegan-calling-twice-in-ios-6