Update bootstrap-wizard.js

I've:
* Refactored
* Reformatted
* Fixed back-button bug (didn't enable on 2 cards)

I was planning on doing more, but then I decided that I should first see if you accept this PR.

I have in fact tested this in pretty much all use-cases and can confirm that I didn't break anything.

I haven't minified this version, as I'd like to leave that up to you. I personally never use the minified version.