Make Bootstrap Improvements Optional

[KazW]

Hi, thanks for creating this library, it makes adding bootstrap to Ember really easy!

That said, I believe the vendor/ui-bootstrap/ui-bootstrap-improvements.css file is best left as an opt-in, instead of being default. It breaks the look of the fixed navbar, putting a 1rem gap between it and the top of the viewport. Right now my solution has been to remove the assets/vendor.css line from app/index.html. This solution is less than ideal since I may need that file in the future, and would have to undo the improvements explicitly in my app's CSS.

I think that for those wanting a stock bootstrap experience, it should be more easily attainable.

Let me know if you agree, if so, I'm happy to do the work and open a PR.

Thanks!