Added a gesture 

I added a gesture, where if you make a fist in front of the leap motion device, then all the displays of the computer will be put to be sleep. I also made appropriate additions to README.md.

I felt like this might be an important feature for the app because if you have any work that you're doing that you don't want anyone to see (important company work, etc.), then you can easily put the displays to sleep by the gesture and now, no one can unlock the computer without a password.