Kunal/touching hidden sprite

Addresses #233, #472. 

If the asker's block is hidden, still returns true if touching.
If the surrounding block is hidden, returns false.
I don't think so...

1. the attribute doesn't exist if the block is never hidden, so we have to distinguish between None and False.

2. If you are hidden and you happen to touch a block that is visible, it still should count as touching (I agree with Brian on this one...)