Fix issues in IE

[backflip]

• Rewriting for … of loops. An alternative would be integrating babel-polyfill (or suggesting the user to load it), but it would increase the payload by 30kb. Selectively polyfilling for Array.from wasn't really successful (still choking on Out-of-date browser support #49 (comment))
• Using Element.parentNode as a fallback for Element.parentElement, which apparently doesn't work for TextNode in IE.
• Using a less elegant approach to make NodeList iterable ([].slice.call(document.querySelector())over [...document.querySelector()])
• Mentioning the need for a Element.classList polyfill in IE9
• Dropping IE7 and 8 (I didn't get it to work there)