favicon and homescreen image

I added a CC0 [robot image from the Noun Project](http://thenounproject.com/noun/robot/#icon-No1616) as the favicon and apple-touch-icon image for when you "Add To Home Screen" on the iPhone. If you merge this you'll need to:

    cd web 
    npm install
    grunt

To copy the image file over to where it needs to be, and then restart marvin.