Mark required questions with asterisk

For #123.

![screen shot 2014-12-11 at 1 29 32 pm](https://cloud.githubusercontent.com/assets/4962198/5402469/cb83f29a-8139-11e4-85d6-f610c70a5b4d.png)

Feature request: Set a tooltip message? E.g. ```req.title = "This question is required"```