Use new golang.org/x/... import paths.

See http://golang.org/s/go14subrepo.

Trim (unnecessary) trailing whitespace.

I've also changed the `golang.org/x/net/html` package link in `README.md` to point to the godoc rather than its source (which can be navigated to from godoc), since that is a more presentable way to view a package. I hope that's okay.