Array.prototype.indexOf polyfill is incorrect

Current `Array.prototype.indexOf` polyfill is incorrect. Our code broke in ie8 because our polyfill came after nunjucks. I replaced it with the one from MDN (with fixed coding-style).

But I believe that it's a bad idea to add this polyfill as it's not what nunjucks is made for. Would you accept a pull request where all `indexOf`'s are replaced with `lib.indexOf`?