close modal through Esc key

Currently, Esc key works on Firefox, but not on Chrome and Safari.

Answers under [this SO thread](http://stackoverflow.com/questions/12630156/how-do-you-enable-the-escape-key-close-functionality-in-a-twitter-bootstrap-moda) suggests `tabindex="-1"`.

But [Mozilla doc](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement.tabIndex) says that `tabindex` should be positive, so I choose `0`.