Revise fruit machine (Issue #44)

Don't just pull this one!

This branch is a significant rewrite and simplification of this project. It addresses (I hope) all the issues listed in issue #44. In particular, the logic for handling the endgame is now much simpler. I also fixed a few typos while I was in there.

Take a look and see what you think. It'll need an editing pass.

The Scratch2 version is at http://scratch.mit.edu/projects/18388780/ A Scratch1 version is in the filetree.