Change `fnBind` to a normal lib function instead of a polyfill

I know [we document it](http://modernizr.com/docs/#prefixeddom), but it’s pretty hidden and [might surprise some people](https://twitter.com/SteveWilshaw/status/448147756755218433). Also it could cause the fn.bind detect to fail (haven’t checked this though) – and generally I don’t think Modernizr should be changing the environment that low down, certainly not without an explicit opt-in.

It was added way back in #478, but that ticket ended up just discussing the API for `prefixed()` and didn’t discuss whether to polyfill or implement as a library function.

Also relevant: [this chap](http://webreflection.blogspot.co.uk/2014/03/do-not-require-polyfills.html) reckons polyfills shouldn’t be AMD dependencies – and I think I agree. Maybe.