Changed the Hang.js file in response to the issue that I had raised

Once a letter is guessed then after that the user can't guess that letter again. This should solve the issue that I had pointed out before!

I am working on a list that is displayed that shows the letters have been already guessed. I will send another pull request regarding that once I am done with it.

Do have a look at Hang.js and if you find that this solves the issue then you can merge it!!