Added an Array.indexOf polyfill for IE8

Microsoft did not implement the **ECMAScript** function for `Array.prototype.indexOf()` until Internet Explorer 9.  
A polyfill is thus necessary for D.js to work in Internet Explorer 8. The polyfill used has been found in this [stackoverflow thread](http://stackoverflow.com/questions/1744310/how-to-fix-array-indexof-in-javascript-for-ie-browsers).