Give progress bar a high z-index value by default

This ensures that the progress bar is less likely to be obscured by other elements on the page. In particular, it fixes a problem where the Turbolinks progress bar gets hidden beneath Twitter Bootstrap's static top navbar (which has a z-index of 1000).

I chose 2000 for the default z-index value based on the example of the [pace.js minimal theme](https://github.com/HubSpot/pace/blob/master/themes/blue/pace-theme-minimal.css#L18), which is another implementation of a very similar progress bar.

[See issue 418](https://github.com/rails/turbolinks/issues/418#issuecomment-60974871) for more discussion.

This is an alternative to #422.