Add MenuState.

Menus for selecting one of a set of following states are pretty common:

```javascript
self.states.add('states:main_menu', function(name) {
            return new ChoiceState(name, {
                question: "Ureport (Speak out for your community)",
                choices: [
                    new Choice('poll', "This week's question"),
                    new Choice('results', 'Poll results'),
                    new Choice('reports', 'Send report')
                ],
                next: function(choice) {
                    return {
                        poll: 'states:poll:question',
                        results: 'states:results:choose',
                        reports: 'states:reports:submit'
                    }[choice.value];
                }
            });
        });
```

Would be nice to be able to write these as:

```javascript
self.states.add('states:main_menu', function(name) {
            return new MenuState(name, {
                question: "Ureport (Speak out for your community)",
                choices: [
                    new Choice('states:poll:question', "This week's question"),
                    new Choice('states:results:choose', 'Poll results'),
                    new Choice('states:reports:submit', 'Send report')
                ]
            });
        });
```