Eliminated colons and added single quotes to list of valid menu choices

I first started using this gem today.  When I tried entering 'x' in the examples/menus.rb example in the last test, I got a list like:

```
:load, :save, :quit, :help
```

Not knowing any better, I tried entering one of those choices literally, including the colon.  That didn't work, but typing it without the colon did.

Omitting those colons in the list viewed by the user would make a right choice more intuitive.  I also added single quotes, but you may think that excessive, and feel free to remove them. It now looks like this:

```
'load', 'save', 'quit', 'help'
```