Changed sticky to create a new stylesheet, rather than inserting rules into an existing one

On firefox, using CSSStyleSheet#insertRule raises a security error if the stylesheets are not hosted on the same domain as the page. This is a problem if you use a cdn to host your assets. Instead, I've made the script insert a new stylesheet into the header with the sticky rules. This works for me on all major browsers

I had problems running the tests - the second one fails for me both on master and on my branch (using safari)