Add a calendar widget to choose meal date

I think this makes choosing the date of a meal easier (the time still
needs something, even if that something is just AM and PM). Obviously we
can define more attractive CSS for the calendar.

![eat-screen](https://f.cloud.github.com/assets/887446/130370/9491173e-7017-11e2-8f87-922b2e50b30f.png)