Add danger class to BootstrapFlashHelper. Fixes #639

Bootstrap 3 renamed `alert-error` to `alert-danger`. See: http://getbootstrap.com/components/#alerts

This adds support for:

```ruby
flash[:danger] = "oopsie daisy"
```

It retains support for `error` flash messages so this will continue to work:

```ruby
flash[:error] = "oopsie daisy"
```

Also, this adds accessibility attributes to the `button` element.
Merge pull request #640 from lancefisher/bootstrap3

Add danger class to BootstrapFlashHelper. Fixes #639